220 ARTHROPODA [CH. 



and acquires a spiral twist as it moulds itself to the interior of its 

 borrowed shell. 



In the Lobsters, Shrimps, etc., which form the division Macrura 

 (Gr. /xaK/oo?, long ; ovpd, tail), the tail is relatively large and is not 

 folded up against the thorax. 



The group is for the most part marine, though the well-known 

 fresh-water Crayfishes, Astacus in Europe and Cambarus in North 

 America, form striking exceptions. 



In the lowest family of the Macrura, the Penaeidae, vestigial 

 exopodites are still retained on the thoracic limbs, and all four 

 series of gills, pleurobranchs, anterior arthrobranchs, posterior 

 arthrobranchs and podobranchs are well developed. These primi- 

 tive Shrimps are very closely related to Euphausia and its allies 

 amongst the Schizopoda. 



Some Decapoda have left the sea and taken to living on land 

 and this has in some cases involved a change of structure, the gills 

 which breathe water being supplemented by the soft vascular 

 lining of the gill-cavity covered by the branchiostegite which, like a 

 lung, breathes air. 



Sub-order 3. Stomatopoda (Hoplocarida). 

 The Stomatopoda (Gr. oro'/xa, a mouth) are a sharply denned 

 group with few genera, and may be regarded as a peculiarly 

 specialised offshoot from the primitive Schizopoda. The members 

 attain a considerable size, some eight inches or more in length. The 

 portions of the head carrying the eyes and antennules have become 

 freely movable. The carapace is small and only covers the anterior 

 five thoracic segments ; the appendages of these segments are turned 

 forward towards the mouth and take part in feeding, and so are 

 termed maxillipedes. They end in a claw, the last joint shutting 

 down on the penultimate one like a knife blade into its handle. They 

 are thus very different from the maxillipedes of Schizopoda or 

 Decapoda, which are really limbs on the way to become jaws and 

 have developed gnathobases. The maxillipedes of Stoma'topoda 

 are grasping, not chewing organs, and have undergone the same 

 modification as the great claw of the lobster, which might just as 

 reasonably be called a maxillipede. A fertile source of confusion 

 in the study of the Arthropoda is the use of names like maxillipede, 

 thorax, abdomen, etc., to denote different things in different groups. 

 The last three thoracic limbs are for walking ; they are very feeble 

 and retain a rudiment of the outer fork or exopodite. The 



