6o 



PHYSIOLOGY OF THE INVERTEBRATA. 



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

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

 The mouth lies behind the mandibles, and is a Avide 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 ca3cum, 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 Astacux 

 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 cascal 

 tubes, whose microscopical 

 structure is represented in Fig. 

 16. There are no other cfecal 

 appendages to the alimentaiy 

 canal of Astacns ; in this re- 

 spect the crayfish differs from 

 the Brachyura and some other 

 Macronra. 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 Astams JluviatiJis. As the 

 stomach of the crayfish (l^'ig. i 5) is far in advance of any 



Fir,. 



14. — Alimkntary Canai. of 



ASTACUS. 



= cardiacpart of stomach. /' = an- 

 terior gastric muscles, c = cardiac 

 ossicle. d = pyloric part of 



stomach. c = pyloric ossicle. 



/ = posterior gastric muscles. 

 .;■■ = so-called " liver." /i = ccecum. 

 / = intestine, k — rectum. 



