318 ENTOMOLOGY FOR MEDICAL OFFICERS 



Subclass Malacostraca. 



This subclass, though it contains plenty of small forms, 

 includes all the large Crustacea, and all those that are found 

 in the food-market. 



The number of segments is usually 21 (or 20 if the 

 region of attachment of the eyes be excluded), but in one 

 little group {Nebalia and allied forms) there are 22 segments. 

 Of the segments 6 are supposed to form the head, 8 com- 

 pose the thorax, and 7 (or 8 in the little order where Nebalia 

 belongs) are abdominal. The abdomen very rarely ends in 

 a caudal fork like that of the Entomostraca. 



The terminal segment never carries appendages, but 

 these are, typically, present on all the other segments. 



The new-hatched young is generally unlike the adult, 

 but is exceptionally a nauplius. 



The Malacostraca are usually distributed in nine orders. 



Order Decapoda. ^ 



This Order (JeVa = ten, and ttoi/? = foot), which comprises 

 Prawns, Lobsters, Hermit-crabs, and Crabs, is so called 

 because the five posterior pairs of thoracic appendages are 

 usually long crawling-legs. 



The body consists of (i) a large cephalothorax, formed of 

 the head-segments, which are indistinguishably fused, and of 

 the thoracic segments, most of which are immovably united, 

 and of (2) an abdomen formed of 7 segments which are 

 usually distinct and separate. 



The cephalothorax is enclosed in a large dorsal shield, or 

 "shell," or carapace. The carapace overlaps the cephalo- 

 thorax on either side, so as to leave a space — the gill- 

 chamber — for the gill-plumes, which are attached to some 

 or all of the appendages of the thorax. Currents of water 

 are drawn through the gill-chambers by peculiarly modified 

 processes of the jaws and foot-jaws and sometimes of some 

 of the legs. These currents usually pass in at the bases of 

 the legs and pass out near the mouth. 



In the typical Decapod every segment except the last 

 carries a pair of appendages, but there are numerous forms 



