Animal Kingdom Unit
Squid Dissection Lab
As part of our continued exploration of the animal kingdom today we will perform a dissection of a common European squid, which is a member of the Class Cephalopoda within the Phylum Mollusk. This lab should introduce you to several of the major tissue types and organ systems found within these animals.
Introduction to the Mollusk Body Organization
Mollusks are soft-bodied animals, more familiarly known as clams, octopods, and snails. They are bilaterally symmetrical, with well-developed digestive, circulatory, excretory, and respiratory systems. A calcareous shell may (e.g., snails, clams) or may not (e.g., slugs, octopods) be present.
Mollusks are closely related to annelids (segmented worms) in their mode of development and type of larva, but they are distinguished from the latter by a lack of segmentation. In addition, the extensive coelom, so important in annelid locomotion, is reduced in size in most mollusks; it is usually restricted to the area surrounding the heart and to spaces within the gonads and kidneys. Most mollusks are slow-moving, creeping animals, but in some instances the organization of the body regions has been modified so that swift movements are possible.
In an evolutionary sense, mollusks are extremely plastic, and they demonstrate highly successful adaptations to a variety of habitats. The considerable adaptive radiation exhibited by members of this phylum is reflected in the varied functional morphology of the group. In numbers of species, they are among the most abundant of all organisms. Although primarily marine, representatives are found in fresh water and on land.
All mollusks can theoretically be derived from a generalized body plan consisting of three main body regions: head-foot, visceral mass, and mantle. The head-foot region is the locomotory and sensory portion of the body, upon which rides the visceral mass containing the excretory, digestive, and circulatory organs. The third component of the body, the mantle, forms a fleshy cover over the visceral mass and a skirt around the foot. It secretes the shell ( in those mollusks that have one), which lies on its outer surface, contains the gills (ctenidia), and itself may function in respiration. The mantle alone might be considered the hallmark of the mollusks, for the flexibility of the mantle in both form and function has contributed greatly to the success of the group.
Dissection of the Squid
As the name indicates (cephalo = head; pod = foot), the head-foot is dominant in cephalopods. These are swift-moving carnivores, and their bodies are quite streamlined. The shell is either not well developed or entirely absent. When present, the shell is internal, except in the most ancient lineages (e.g., Nautilus).
Examine a preserved specimen of the squid. Note that there is no external shell and that the major part of the body is enclosed in a soft, fleshy mantle and is sharply demarcated from the rest of the squid body by the collar. Opposite the pointed end of the animal (the apex), you will find the head-foot region. The eight arms and two longer tentacles are derivatives of the foot and surround the mouth. Turn back the arms and tentacles to reveal a muscular membrane running from their bases to the mouth. This membrane is composed of an outer lobed buccal membrane with suckers on its inner surface and an inner peristomial membrane. Notice the horny beak protruding from the mouth. Locate the large, well-developed eyes and the fold of tissue behind the eye, called the olfactory crest.
Orientation at this point may cause problems, so align your specimen in such a way that the following descriptions will not be confusing. Place the animal so the apex is furthest from you and the arms are closest to you. Turn the animal so that the siphon (the tubular projections between the collar and the head-foot region) is facing you. It too is a derivative of the foot. The apex, which bears the visceral mass (digestive organs), is dorsal and the arms are ventral. The surface facing you is posterior. The eyes are on the left and right sides of the body, and the head structures have moved to a position that is dorsal to the foot. Contrary to popular belief, the squid moves in the direction of its tentacles and arms. The fin is the trailing end of the squid when it is motion and acts like a rudder to help them steer to change right and left directional movements.
The mantle is a fleshy cover that incompletely surrounds most of the organs of the body. The space between the fleshy mantle lobes is called the mantle cavity. The openings of the digestive, excretory, and reproductive systems are all found within the mantle cavity. The mantle also functions in secretion of the shell. However, the most distinctive function of the mantle cavity is to provide a space for housing the delicate ctenidia (gills), thus protecting them from the hazards of the environment and permitting an oriented flow of water across them.
Primitive cephalopods show a chambered shell (e.g., Nautilus), but in others the shell is internal or absent. In Loligo, the chitinous endoskeletal element is known as the pen. You will be able to feel the stiff pen running the full length of your animal under the dorsal anterior surface. Of what advantage might a reduced or internal shell be to cephalopods?
The most obvious organs of the mantle are the paired ciliated gills or ctenidia. Strictly speaking, a ctenidium is a respiratory structure that includes ciliated filaments. The ciliary tracts on the filaments draw respiratory water currents into the mantle cavity on either side. The two currents filter through the ctenidial filaments, meet dorsally, and pass backward as an exhalent stream out of the mantle cavity. Gland cells on the filaments secrete mucus, and particles brought into the mantle cavity with the respiratory current are filtered out. Tracts of cilia on the ctenidia direct rejected material toward the midline.
Cephalopods are active predators, streamlined for quick motion; they therefore require a streamlining of their respiratory current. The walls of the mantle cavity are highly muscularized, and movement of water within the cavity is no longer dependent on ciliary action. With the ventral side of the squid facing toward you, observe the construction of the mantle collar and its relationship to the siphon. The siphon is derived from the foot. In life, the mantle cavity expands by muscular action, water enters, and then the collar locks tightly against the head, leaving the siphon as the only exit pathway from the mantle cavity. The siphon is well equipped with muscles and can be pointed for making directed jet-propulsive movements.
Open the mantle cavity by making an incision that runs the entire length of the posterior surface from siphon to apex. Dissect with care so as not to disturb the internal organs. Notice that the mantle consists of a thick layer of circular muscles surrounded by integument, with dorsolateral muscularized extensions, the fins. Turn the cut mantle edges laterally and pin them out to expose the internal organs clearly. Note that the tip of the siphon has a valve that regulates the outflow of water from the mantle cavity. The inner side of the mantle has cartilaginous ridges that keep the inhalent currents separate from the exhalent. There are two ctenidia (gills) oriented so that the inhalent streams pass over each, then converge and exit as a single exhalent stream.
The paired excretory organs of the mollusks are closely associated with the heart. There are two kinds of molluscan excretory organs: brownish pericardial glands (difficult to see in most specimens) and renal nephridia or kidneys. In the kidney, nitrogenous wastes (urea, amines, and predominantly, ammonia) are extracted from the blood. Inorganic substances may be taken back into the blood, and the composition of the waste is determined by the extent of renal secretion and reabsorption. Excretory material is passed to the mantle cavity and voided in an excurrent blast through the siphon.
The cephalopod kidney is derived from two separate renal organs that have fused. The kidney is closely associated with the gill hearts, which are swellings of the blood vessels at the base of the gills. The contractions of the gill hearts forces fluid through the walls of the blood vessels into the kidney.
If necessary, in your opened Loligo, remove the thin semi transparent skin covering the organs of the visceral mass. Locate the rectum ending in the anus at the base of the siphon. If your specimen is a female, there will be two large white nidamental glands that covers most of the internal organs. These will need to be carefully removed in order to see the stomach and other organs. Notice the swollen branchial heart at the base of the left gill and attempt to look for it's close attachment to the kidney. This may be difficult to locate but give it a try.
Mollusks do not have a spacious body cavity (coelom), and in all except the cephalopods, the body space is composed of large sinuses, which act as pooling places for the blood. For this reason, the body space is more accurately described as a hemocoel; (a membranous sac enclosing the heart) and the cavity of the gonad. Blood that has collected in the sinuses from various parts of the body passes first through the kidney and respiratory organ and then via the branchial artery to the heart. The main propulsive force for distribution of blood in the connective tissue spaces gives the blood an additional function to that of distribution; namely, it acts as a component of the hydrostatic (fluid supported) skeleton. The foot of clams, for example, is enlarged and forced outward by the influx of blood and is withdrawn by contraction of longitudinal muscles.
The active swimming and carnivorous habits of the cephalopods require a more efficient circulatory system than in a clam - one that is closed and contains capillaries.
Mollusks demonstrate all possible feeding habits (herbivorous, carnivorous, and omnivorous), and the structure and function of the gut are related to the type of food eaten. Cephalopods are voracious carnivores and do not depend upon ciliary currents for the capture of food. They do, however, retain some of the structural components of the gut, such as the radula (a tooth-bearing, food-gathering structure unique to mollusks).
The mouth is surrounded by eight pointed arms and two longer tentacles. Observe the structure of these arms and tentacles and the arrangement of the suckers upon them. Study the organization of the suckers under the dissecting microscope. In life, the prey is grabbed by a rapid extension of the two tentacles and brought toward the mouth, where it is held firmly in place by the eight arms and killed by an injection of poison. The arms will then hold and manipulate the food while the squid devours its prey. Remove the siphon, and, by median incision, cut into the head, separating the eyes and exposing the buccal mass. This is a muscular organ that bears two horny beaks, which are used for ripping prey. Pry open the beak and observe the radula. Posterior to the buccal mass are a pair of salivary glands, which pour their poisonous secretions into the buccal cavity. Trace the thin-walled esophagus (surrounded by the liver) from the buccal mass to the thick-walled stomach. The stomach emerges from the liver tissue to form the caecum, which extends to the tip of the visceral mass. The liver consists of paired digestive glands fused in the midline; it is a triangular organ with the base located ventrally near the collar. A U-shaped pancreas lies anterior to the stomach; its duct unites with that of the liver before passing into the caecum. The intestine runs forward from the stomach, shows a diverticulum (the ink sac), and terminates in the rectum.
The structure of the nervous system in mollusks ranges from a simple "ladder" type to a system in which there is extreme fusion of ganglia, forming a true brain. The ultimate in invertebrate cephalization is seen in the cephalopods. Evolutionary fusion of ganglia followed by extensive differentiation makes it difficult to compare the ganglia with those of other mollusks. In your specimen, the easiest part of the nervous system to find are the large stellate ganglia, located on the inner dorsal surface of the mantle at the level of the tip of the ctenidia. The stellate ganglia are the motor centers of the mantle and give rise to its giant fiber system, which is favorite material for neurophysiologists.
From the complex structure of the cephalopod nervous system, one would expect these organisms to be capable of a variety of neuromuscular activities. They are particularly well demonstrated by the feeding and swimming movements of the squid. In Loligo, which is a rapid swimmer, the giant fiber system of the mantle is effective in producing quick-firing, rapid muscular contractions. In addition to moving quickly by use of the funnel system, when the squid feels threatened, it may release ink from its ink sac into the water as a way to confuse a larger predator. It will use this opportunity to then swiftly swim away to safety.
The eyes of cephalopods are the best developed among all mollusks, and perhaps among all invertebrates. Carefully remove an eye from your specimen by cutting the eye muscles and optic nerve. The outermost covering is the false cornea, which is underlain by the true cornea. Remove the corneas and identify the spherical lens, the colored iris and choroid coat, and the innermost lining, the retina. How does the organization of the eye of the squid compare with that of the human? In simple terms, the eyes of the squid are almost identical to the eyes of the human. Other than the false cornea, all of the other structures of the eye of the squid are the same as the structures in a human eye.
In cephalopods, the sexes are always separate; the gonad is at the apex of the body and its ducts open directly into the coelom. Fertilization is internal, and there may be complicated courtship behavior and parental care of the young. You should be sure to view the reproductive organs of both a male and a female squid.
If your specimen is a female, identify the single large ovary at the apex of the visceral mass. Identify the large, white nidamental glands, together with the orange-speckled accessory nidamental glands beneath their ventral ends. The oviduct is a transparent tube, possibly packed with eggs, leading by a small ciliated funnel from the vicinity of the ovary. The oviduct loops ventrally, dorsally, and ventrally again as the glandular, thicker walled oviducal gland. It terminates in a flared opening. The large, yolky eggs are shed from the ovary into the coelomic cavity and are picked up by the ciliated funnel of the oviduct. As the eggs pass along the oviduct, they receive an elastic membrane from the nidamental glands and a gelatinous coat from the oviducal glands.
If your specimen is a male, remove the left gill and branchial heart. The single large, white testis is located at the apex of the visceral mass. It opens directly to the coelom by a slit at its anterior end. Near the opening lies the ciliated opening to the vas deferens, an opaque, white, coiled tube leading ventrally between the spermatophoric sac on the right and the thick-walled spermatophoric organ on the left, and opening to the exterior by the penis to the left of the rectum. The penis is not a muscular intromittent organ and is no more than the end of the vas deferens.
Click HERE for the Squid Lab Companion (Identifies what material from the squid dissection will be on the lab exam.)