Class Crustacea

General Morphology and Organ Systems

The body of representatives of this class is traditionally divided into two main tagmata: the cephalothorax and the abdomen (which is most often segmented). Development can be either direct or indirect (with a larval stage).

An interesting feature is the arrangement of the appendages — in the basic plan, they are present on every body segment and are narrowly specialized. Depending on their localization, the appendages perform the functions of locomotion, swimming, respiration, food capture, defense, or act as sensory organs.

Internal Structure:

• Respiratory system: represented by gills, which are anatomically limb outgrowths. In the river crayfish, for example, gills are preserved directly on the walking legs. Oxygen is transported from the gills to the tissues by the blood.
• Circulatory system: of the open type (includes the heart, arteries, and a system of sinuses and lacunae).
• Excretory system: consists of one or two pairs of modified metanephridia — antennal (green) glands. Anatomically, the organ includes a coelomic sac from which a convoluted glandular canal extends, emptying into a reservoir (bladder). The excretory pore is located on the basal segment of the antennae.
• Digestive system: clearly differentiated into three sections: the foregut (with the esophagus and stomach), the midgut (into which the ducts of the paired "liver" empty), and the hindgut, ending in the anus.
• Nervous system: built according to the classical arthropod type. It includes paired supraesophageal and subesophageal ganglia, as well as a ventral nerve cord.
• Reproductive system: gonads and their ducts have a similar structure in male and female specimens, though the location of the genital openings differs.

Subclass Entomostraca

This group unites many species of small planktonic crustaceans inhabiting primarily the water column of freshwater bodies (ponds, lakes). They feed on amoebas, infusoria, and small plankton. Development proceeds with incomplete metamorphosis.

Key Anatomical Differences:

• Appendages are located only on the cephalothorax.
• The abdomen is completely devoid of appendages and ends in a specific fork-like structure — the furca.
• Adult specimens possess an unpaired eye.

Medically Significant Orders:

1. Cladocerans (Cladocera). A typical representative is Daphnia, or the "water flea" (Daphnia pulex). Its body is covered by a transparent bivalve carapace that leaves the head free. A single compound faceted eye is located on the head. Daphnia move in characteristic jerks due to powerful strokes of their antennae. Daphnia can serve as intermediate hosts for the broad fish tapeworm.
2. Copepods (Copepoda). These include cyclops (Cyclops sp.), eucyclops, diaptomids (Diaptomus sp.), and eudiaptomids. Unlike daphnia, they lack a carapace. Their cephalothorax is dorsoventrally flattened, bearing a single simple eye and two pairs of antennae, with the first pair (antennules) consisting of numerous small segments and being exceptionally well-developed. Copepods serve as obligatory intermediate hosts for the broad fish tapeworm (Diphyllobothrium latum) and the guinea worm (Dracunculus medinensis).

Subclass Malacostraca

This subclass is distinguished by a colossal diversity of forms: from microscopic woodlice to huge lobsters, spiny lobsters, crabs, and river crayfish. Unlike their lower relatives, higher crustaceans have appendages on absolutely all body segments.

The abdomen bears 6 pairs of swimming legs, and its last pair has transformed into specialized structures — uropods. Most species inhabit marine and freshwater environments, but terrestrial forms also exist (woodlice, as well as certain crab species, such as the coconut crab — Birgus latro).

Significance in Nature and Medicine:

• Sanitary role: many large crustaceans are scavengers. By feeding on animal carcasses, they actively cleanse water bodies of decomposing organic matter.
• Epidemiological role: freshwater crayfish (e.g., the river crayfish Astacus fluviatilis) and various crab species serve as intermediate hosts for a dangerous parasite — the lung fluke (Paragonimus westermani). Infection foci are localized in Korea, Japan, parts of China, India, Southeast Asia, and the Russian Far East. The infectious stage of the parasite (metacercaria) forms directly in the gills or muscles of the crustaceans. Humans become infected alimentarily by consuming their meat without adequate heat treatment.

Class and exam questions

What else to study on this topic

• The life cycle of the lung fluke (Paragonimus westermani) and the process of metacercaria formation in crustacean tissues.
• The transmission mechanism of the broad fish tapeworm (Diphyllobothrium latum) via copepods and daphnia.
• Comparative morphology and evolution of crustacean appendages in relation to their functions.
• Detailed structure of the antennal (green) glands and the filtration mechanism of metabolic wastes.
• Habitat and ecological role of terrestrial crabs (e.g., the coconut crab).

Other topics in the "Medical Arachnoentomology" section

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