6o 



PHYSIOLOGY OF THE INVERTEBRATA. 



HIND 

 GUT 



As an example of the Macroura, we describe in detail the ali- 

 mentary canal of Astacus fluviatilis (the fresh-water crayfish).' 

 The mouth lies behind the mandibles, and is a wide aperture 

 bounded by the labrum in front and the metastoma behind. 

 This oral aperture leads into a wide but short oesophagus 

 situated on the ventral side of the head. The oesophagus 

 opens almost vertically into a large stomach divided into 

 cardiac and pyloric portions. The pyloric portion, dilated 



dorsally in a c^cum, passes 

 directly into a long tubular 

 intestine, which dilates into a 

 small rectum, and finally ter- 

 minates in an anus (Fig. 13). 

 The only lateral appendage to 

 the alimentary canal of Astacus 

 is the so-called liver (Figs. 14 

 and 16), whose ducts open on 

 each side of the pylorus. This 

 so-called liver is in reality a 

 digestive gland or pancreas, 

 and consists of numerous ceecal 

 tubes, whose microscopical ; 

 structure is represented in Fig. 

 16. There are no other csecal 

 appendages to the alimentary 

 canal of Asfactos ; in this re- 

 spect the crayfish differs from 

 the Brachyura and some other 

 Macroura. But it may be 

 stated that " in many Crustacea the digestive canal is sur- 

 rounded by cells filled with oily or fatty matter of a yellow 

 or blue colour ; they may be compared to an omentum, and 

 probably serve as a store of nutriment, to be drawn upon 

 during the moult, or when food is scarce." 



There are no salivary glands in Astacus fiuviatilis. As the 

 stomach of the crayfish (Fig. 15) is far in advance of any 



Fig. 14. 



-Alimentary Canal 

 Astacus. 



'. = cardiac part of stomach. /; = an- 

 terior gastric muscles, c = cardiac 

 ossicle. d = pyloric part of 



stomach. e = pyloric ossicle. 



/ = posterior gastric muscles. 

 g = so-called *' liver." h = caecum. 

 i = intestine, k = rectum. 



