14 THE CRUSTACEA 



Branchiae. — In many of the smaller Crustacea there are no 

 special branchiae, and respiration is carried on by the general sur- 

 face of the body. When present, branchiae are usually formed by 

 differentiation of parts of the ajipendages, often the epipodites, 

 but the shell-fold has probably in many cases a respiratory function, 

 and processes from its inner surface (Cirripedia) or from the 

 surface of the body (some Ostracoda) may develop as branchiae. 

 In the more primitive of the Malacostraca, the gills are formed 

 by the epipodites of the thoracic limbs (podobranchiae), and this 

 was probably also the original nature of those branchiae which, in 

 the Decapoda, are attached to the articular membrane between the 

 limb and the body (arflirohranchiae), or to the body -wall itself 

 {pleurohrancUiae). In the Isopoda the respiratory function is 

 assumed by the lamellar rami of the abdominal appendages. 



Many terrestrial Crustacea have no special adaptations for 

 aerial respiration. In land-crabs of different families, however, 

 the linine: membrane of the branchial chamber is covered Avith 

 vascular papillae and acts as a lung. Still more remarkable are 

 the breathing organs of many of the terrestrial Isopoda or Woodlice. 

 These are ramified tubular invaginations of the integument in 

 the abdominal appendages, and are precisely analogous to the 

 tracheae of other air-breathing Arthropoda. 



Alimentary System. — In the great majority of Crustacea the 

 alimentary canal is nearly straight, except at its anterior end, 

 Avhere it curves downwards to the ventrally placed mouth. The 

 only cases hitherto described in which it is actually coiled upon 

 itself are in certain Cladocera and in a single genus of Cumacea. 

 As in other Arthropoda, it' consists of stomodaeum, mesentero)/, and 

 procfodaeum, the first and last with a lining of chitin continuous at 

 mouth and anus with the exoskeleton. The relaj-ive proportions 

 of these three divisions vary greatly, and the extremdi* abbreviation 

 of the mesenteron found in the common Crayfish is by no means 

 typical of the Class. Even in the closely related Lobster this 

 section of the gut may be several inches long. 



The whole length of the alimentary canal is provided, as a 

 rule, with circular and longitudinal muscle -fibres running in its 

 walls, and there are often also muscle-bands extending to adjacent 

 portions of the body-wall. In the anterior part of the stomodaeum 

 these muscles are more strongly developed to perform the move- 

 ments of deglutition. In a few Branchiopoda and Ostracoda the 

 chitinous lining of this region develops spines and hairs Avhich 

 help to triturate and strain the food, and in some Ostracoda the 

 armature assumes a more complex form as a series of toothed 

 plates moved by special muscles. It is among the Malacostraca, 

 however, and especially in the Decapoda, that this aj)paratus, the 

 so-called " gastric mill," reaches its greatest complexity. It con- 



