XII] ASTACUS 189 



endoderm is reduced to exceedingly small dimensions and yet, from 

 experiments made on other Crustacea, we have every reason for 

 believing that it is this part alone which is capable of digesting 

 and absorbing nourishment. Hence it comes about that the nourish- 

 ment must be supplied to it in the form of the finest powder, and 

 so a Crustacean and the same is true more or less of all Arthro- 

 poda is quite incapable of either swallowing its prey whole or 

 even of bolting large morsels of food. 



From the mouth the so-called oesophagus ascends upwards and 

 slightly forwards till it expands into the so-called stomach, which is 

 divided by a deep transverse indentation into an anterior "cardiac" 

 portion and a posterior "pyloric" portion, these names being sug- 

 gested by fanciful analogies with parts of the human stomach. Both 

 oesophagus and stomach are simply portions of the stomodaeum 

 and of course like the rest of the ectoderm are covered with cuticle 

 internally. In the case of the stomach this cuticle is thickened in 

 places so as to form firm sclerites some of which have tooth-like pro- 

 jections which are brought together by the action of muscles attached 

 to the stomach and so constitute a mill by means of which the food 

 which has already been torn into fragments by the gnathites is 

 ground up into a powder. This powder is sifted through a sieve of 

 setae which guard the entrance to the pyloric sac ; in this way only 

 the finest grains reach the short portion of the intestine lined by 

 endoderm. The muscles which cause the grinding action of the 

 stomach are two pairs, termed the anterior gastric and poste- 

 rior gastric respectively. These are longitudinal bands arising 

 from the skin under the carapace and inserted in the sclerites in the 

 stomach wall. The anterior gastric muscles arise from the anterior 

 part of the carapace and are attached to a transverse sclerite in the 

 upper wall of the cardiac portion of the stomach termed the cardiac 

 ossicle, whilst the posterior gastric muscles arise from the posterior 

 part of the carapace and are attached to a sclerite called the pyloric 

 ossicle which runs transversely in the upper wall of the pyloric 

 portion of the stomach. The action of these muscles is to drag 

 asunder the cardiac and pyloric ossicles and to stretch the wall of 

 the stomach between them. When the muscles relax the elasticity 

 of the lining of the stomach restores it to its former shape. With 

 each end of the cardiac ossicle there articulates a sclerite called 

 the pterocardiac ossicle which curves downwards and backwards 

 along the side of the stomach, and similarly with each end of the 

 pyloric ossicle a zygocardiac ossicle articulates which curves 



