Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Metazoa
Arthropods
Subphylum Branchiopoda - Class Crustacea

The Class Crustacea comprises about 35,000 species, predominantly aquatic organisms (hydrobionts). The overwhelming majority of these forms inhabit the marine environment, while some have adapted to life in freshwater bodies. Among them, some lead a benthic lifestyle, whereas others constitute an essential component of the plankton. Only a few species, such as woodlice and terrestrial crabs, are capable of living on land. Parasitic forms are also found within the crustacean group. Crustaceans differ from other Arthropods by the presence of 2 pairs of HEAD appendages—antennules and antennae—as well as the retention, in many cases, of primitive biramous limbs. The body length of crustaceans ranges from fractions of a millimeter to 80 cm. Many species serve as food for fish and whales, while others are harvested by humans for their delicious and valuable meat. The crustacean body consists of 3 tagmata (regions): the head, Thorax, and abdomen (Fig. 46).

The head is formed by the fusion of 5 segments. Accordingly, it bears 5 pairs of appendages: 1) antennules—uniramous tactile feelers; 2) antennae—the 2nd pair of feelers; 3) mandibles; 4) the 1st pair of maxillae; 5) the 2nd pair of maxillae.

In A number of crustaceans, the head is movably connected to the thoracic region, whereas in other forms these body parts fuse to form a unified cephalothorax. In many species, the dorsal cuticular coverings of the head expand and cover the thorax, forming a continuous carapace.

Fig. 46 DORSAL VIEW OF a crayfish and its appendages:

1 — antennule; 2 — spine; 3 — antenna; 4 — cheliped (claw); 5 — eyes; 6 — carapace; 7 — gill cover; 8 — cephalothorax; 9 — branchiocardiac grooves; 10 — abdomen.

The thoracic region of the crayfish, crabs, and certain other large crustaceans is formed by the fusion of 8 segments. In small, low-organized forms (such as *Daphnia*, *Cyclops*, etc.), the number of thoracic segments ranges from 2 to 12 or even more, often freely articulated with one another. The crustacean segments making up the thorax bear appendages that can perform various Functions—locomotion, Respiration, and food delivery to the Mouth. In the latter case, they are referred to as maxillipeds. Highly organized crustaceans possess 3 pairs of maxillipeds. Gills are located on the basal segment of the 2nd and 3rd pairs of maxillipeds. The 4th pair of appendages is modified into a grasping organ terminating in a pincer, while the remaining limbs, from the 5th to the 8th pair, are typical walking legs (Fig. 47).

The abdomen of highly organized crustaceans consists of 6 movably articulated segments, each bearing a single pair of abdominal appendages (pleopods). In lower forms, the abdomen may contain anywhere from 1 to several dozen distinct segments typically lacking appendages.

The abdominal legs of the crayfish have a biramous Structure. In the male, the 1st pair is modified into a copulatory organ and appears as tubular structures. In females, the 1st pair of pleopods is reduced. The 6th pair of crustacean limbs has transformed into flat swimming paddles. Together with the telson (the terminal segment of the abdomen), they form a fan-shaped swimming apparatus that allows the animals to swim backwards.

Crabs often exhibit a peculiar defense mechanism—the spontaneous shedding of a limb, which can occur even upon slight irritation. This self-mutilation, or autotomy, is associated with a highly developed capacity for regeneration. A new limb grows in place of the lost one.

The chitinized cuticle of crustaceans is impregnated with a significant amount of calcium carbonate, imparting high rigidity. Variations in the thickness of this hard integument ensure the mobility of the body and limbs. Dense plates on the dorsal and ventral sides cover each segment, whereas along the boundaries between them, thin and soft Chitin forms folds that straighten out during flexion. The chitinous parts at the limb joints are arranged similarly. On the inner side, the hard integument forms numerous ridges and bars to which Muscles attach. Thus, the integument of crustaceans functions simultaneously as an exoskeleton.

Fig. 47

Appendages of the crayfish:

1 — antennule; 2 — antenna; 3 — mandibles; 4, 5 — 1st and 2nd pairs of maxillae; 6–8 — maxillipeds; 9–13 — walking legs; 14–19 — pleopods; 20 — gills.

Fig. 48 Internal Structure of the crayfish:

A — dissected crayfish; B — dissected gastric mill (Stomach) of the crayfish; 1 — rostrum; 2 — gastric mill; 3 — digestive gland (Liver); 4 — intestine; 5 — Heart; 6, 7 — Arteries; 8 — Ovary; 9 — ventral nerve cord; 10 — gills.

The Digestive System of crustaceans, as in all arthropods, consists of 3 parts: the foregut, midgut, and hindgut (Fig. 48). The paired digestive gland, commonly referred to as the liver, is well developed. In the crayfish, the mouth is located on the ventral side of the head. A short Esophagus extends upward from it, opening into The Stomach, which is divided into 2 regions. In the first region, food is crushed and ground up, whereas In the second, it is partially digested and filtered. The finely ground and filtered food enters the short midgut and the digestive gland, where final Digestion AND ABSORPTION take place. Undigested residues pass from the midgut into the hindgut and are subsequently expelled through the anus.

Respiration in large crustaceans is carried out by gills. In the crayfish, these are covered by the lateral PARTS OF THE carapace. Many small crustaceans breathe through the general body surface, as they lack gills.

The Circulatory system OF crustaceans is open. The Heart is located in the pericardial sinus on the dorsal side. It pumps Blood either into Blood Vessels or directly into the body cavity. After delivering oxygen to the Tissues and picking up carbon dioxide, the blood collects in a venous vessel leading to the gills, where gas exchange occurs.

The excretory Organs of crustaceans are a pair of green glands. Each gland consists of several consecutive canals. The last of these empties into a Urinary Bladder, which opens via an excretory pore located either at the Base of the antennae or at the base of the maxillae.

The Nervous system of crustaceans consists of a supraesophageal ganglion and a connected subesophageal nerve mass, from which the ventral nerve cord extends, featuring two variously spaced nerve cords depending on the species.

Crustaceans achieve a high level of development in their organs of Vision, Touch, equilibrium, and chemical sense, or Olfaction. Compound (facet) eyes consist of A large number of rather complex individual ommatidia grouped together. Each ommatidium captures a portion of the field of view. Consequently, animals possessing such eyes produce a mosaic image, as if viewing the world through a fine grid.

The overwhelming majority of crustaceans are dioecious (have separate sexes). Furthermore, many species exhibit pronounced Sexual Dimorphism. For instance, the female crayfish differs from the male by a broader abdomen and the absence of the 1st pair of pleopods. In many lower forms, males are significantly smaller than females.

The Development of most crustaceans involves several larval stages. In the crayfish, however, development is direct: fully formed juvenile crayfish hatch from fertilized eggs carried by the female on her abdominal appendages. Direct development is characteristic of only a small number of species

Fig. 49 Representatives of the class Crustacea:

1 — daphnia; 2 — cyclops; 3 — woodlouse; 4 — Water hoglouse; 5 — common shrimp; 6 — red king crab.

The class Crustacea comprises over 15 orders. Representatives of some of these orders are well known and play a significant role in human practical activities (Fig. 49).