116 UNDERWATER GUIDE TO MARINE LIFE 



MALACOSTRACANS: Order Malacostraca 



The rest of the crustaceans, including crabs, lobsters, sand fleas, shrimp, 

 prawns and others, are included here. In all of them, the appendages on difi^er- 

 ent parts of the body are sharply marked off from each other. The largest 

 appendages are those of the trunk, which are used for locomotion and food- 

 getting and very often bear claws. There are almost always stalked eyes on 

 the head. A carapace covers the head and at least part of the trunk. 



All members of this group may be referred in general body plan to a 

 central type, that of the shrimp or prawn, in which the body is adapted mainly 

 for swimming. The species that depart from this general body plan do so as a 

 result of a change of habits to burrowing or crawling, which causes a modification 

 of the limbs on which heavy claws may be developed. 



ISJehalia, a little crustacean common all over the world on shallow rocky or 

 weedy shores (especially near foul water), is the most primitive of this group. 

 It reaches only a half-inch in length. 



The amphipods are elongated and compressed little crustaceans without a 

 true carapace. These are the beach fleas or sand fleas, and there are great num- 

 bers of them found literally all over the world, living either semiterrestrially on 

 moist beaches or in shallow water. Gammarus is one common genus. The 

 amphipods can swim, walk, and jump. They rarely reach an inch in length, 

 but sometimes they grow larger. The coloration of amphipods often matches 

 that of their background. Except for their movement, they are often difficult to 

 see. Amphipods form a major item of diet for many fishes. One interesting 

 amphipod, the skeleton shrimp, CapreUa, is about a half-inch long and is often 

 seen clinging to seaweeds, especially AgardhieUa, or other suitable substrate 

 by means of its small, posteriorly placed legs. It captures small prey with clawed 

 legs near the mouth in a comical-looking, lurching motion of the whole body. 

 Its acrobatics as it clambers over seaweeds are very amusing to watch. 



The isopods are small crustaceans that are depressed in body form. The 

 terrestrial sow bugs and pill bugs are examples. Isopods are found on land, 

 in fresh water, over almost all of the sea, and are even sometimes parasitic. 

 Therefore, they qualify as the most widespread of all crustacean groups. Most 

 species are small and found creeping over rocks, in seaweed, or over sand. 



The mantis shrimps, Squilla, are similar to isopods in being flattened, but 

 they live in burrows in soft bottoms and bear large, raptorial claws, like those 

 of the insect praying mantis, with which they capture prey. Some mantis 

 shrimp reach the large size of almost a foot, but most species are under a third 

 of that size. They are found all over the world. 



The rest of the malacostracans are grouped under Eucarida, which includes 

 all the crustaceans in which the carapace covers the whole head and trunk 

 and forms a single unit called the "cephalothorax." Krill, or euphausians, are 

 pelagic, small eucaridans that occur in huge swarms in cold seas and form a 

 basic item of the baleen whale's diet. 



The largest of the eucaridans are the decapods, or species with ten legs, 

 which include the crabs, lobsters, shrimps, and prawn. Some decapods are 

 armed with large claws and some are not. Among those that do not have 

 claws is the sand bug, Hifpa, which lives a rough and tumble life in sandy 

 shores, where it constantly has to battle the force of the breaking surf. The 



