PHYSIOLOGY OF THE INVERTEBRATA. 



57 



the pancreas (the so-called liver) discharge the secretion into 

 the anterior part of the alimentary canal. There are in some 

 oi the Am 2:)hipo<J a Malpighian tubules which oi)en into the 

 posterior part of the alimentary canal.* 



(29) The Isopoda. — In this order, to which the wood-louse 

 belongs, the body is usually broad, depressed, or vertically 

 flattened, and more or less arched. The alimentary canal is 

 similar to that of the AmphijwrJa. 

 Fig. 12 represents the digestive 

 system of Oriiscus (the wood- 

 louse). It forms a straight tube, 

 the masticatory portion being 

 strongly armed. Two ducts, 

 leading from a pair of cellular 

 pancreatic follicles on each side 

 of the alimentary canal, pour the 

 digestive fluid into the anterior 

 portion of the canal. The num- 

 ber of these follicles is variable 

 in other genera of the Isopoda, 

 but in Oniscus there are always 

 four, two on each side of the 

 alimentary canal. Sometimes 

 there are one or two tubules which 



open into the posterior part of the intestine. The function 

 ot these appendages is of the same nature as the Malpighian 

 tubules of the Inseda. 



(30) The Sfomapoda are elongated. Cnistarca having a 

 short, cephalo-thoracic shield which does not entirely cover 

 all the thoracic segments. In the genus Sqnilla there are 

 five pairs of maxillipeds, and three pairs of backwardly 

 turned biramous thoracic feet. The alimentary canal, some- 

 what dilated in its anterior part, is a long cylindrical tube, 



^cU 



Fig. 12. — Alimentary Canal 

 OF Oniscus. 



a — masticatory portion. 

 /' = pancreatic follicles. 

 (• = intestine. (/ = anus. 



* Many new species of the Amjihipoihi liave been described by Bars in 

 llie t'ltriitiania Vldensk. i^e'al;. ForhaviV., 1882, No. 16, pp. 75-115. 



