ALIMENTARY CANAL. 351 



of a stridulating apparatus by some Decapoda (Ocypod Crabs 

 and the Hermit Crabs Coenobita), which live largely in air,* 

 that a corresponding auditory organ must be present some- 

 where in the body ; and the possibility is not excluded that 

 the sacks in question have, as in Vertebrata, a double function, 

 informing their owner of the presence of audible vibrations, 

 and also of their own relation in space. 



The alimentary canal consists of fore- mid- and hind-guts, 

 of which the first and last are lined by chitin and derived re- 

 spectively from the stomodaeal and proctodaeal involutions 

 (ectodermal) of the embryo, while the mid-gut is endodermal. 

 The remarkably small extent of the mid-gut in the Isopoda 

 and some Decapoda, and the corresponding increase in length 

 of the hind-gut have been already mentioned (p. 340). 



The fore-gut is usually a short and simple tube in the Ento- 

 mostraca, but in most Malacostraca its posterior region is modi- 

 fied into the masticatory stomach. Salivary glands are usually 

 wanting. From the upper end of the oesophagus the alimentary 

 canal usually runs straight to its termination, though it is 

 coiled in some Cladocera, and Caiman finds that the Cumacean 

 Platycuma Holti has a tract of the gut (? fore-gut) coiled. The 

 mid-gut is marked off by constrictions anteriorly and posteriorly ; 

 in front it is produced into diverticula, which are usually called 

 the liver, but as their walls contain ferment cells the term hepato- 

 pancreas probably more nearly expresses their function. They 

 may be paired (Fig. 251) or unpaired in the Entomostraca, and 

 in the Malacostraca are represented by one to four pairs and 

 they may remain simple, or be subdivided as in Decapods. In 

 Stomatopods it has recently been shown by Orlandi f that there 

 are two of these diverticula opening into the anterior part of 

 the mid-gut, lying parallel with the alimentary canal, and 

 expanding into voluminous lateral branches in the posterior 

 segments of the body including the telson. The supposed 

 segmental and hence exceptional relation of these branches 

 with the gut in this group is thus shown not to exist. 



In some Amphipods a single or paired caecum is present at the 



* Cf. Alcock, Ann, and Mag. of Nat. Histy., ser. 6, vol. 10,' p. 336. 



f S. Orlandi, Sulla Struttura dell' intestine della Squilla mantis 



Rond., Atti della Soc. Ligustica di Sc. nat. e geog, vol. xii. N. 2 (1901), 

 p. 112. 



