242 ANIMAL BIOLOGY. [Part II. 



diagram. Posteriorly it gives off a superior abdominal artery 

 (s. a. a.), which runs along the dorsal surface of the intestine 

 (int.), which takes a straight course along the median line to 

 the telson. The coiled tubes on either side of the heart are the 

 vasa deferentia (v. d.), and the masses of brown caeca belong to 

 the digestive gland or so-called liver (a. d. g., p. d. g.). 



In the female there is in place of the testis an ovary of some- 

 what similar form, but shorter. In the breeding season it is 

 large and mottled with berry-like eggs. The oviducts are 

 straight thin-walled tubes running nearly vertically downwards. 



The muscles of the crayfish are white, and the blood colourless. 



The Alimentary System. The crayfish is a voracious feeder. 

 The food is seized by the forceps, transferred to the chelate 

 legs, which tear it to pieces and then pass it on to the maxilli- 

 peds and maxillae, the exopodites of which are in constant 

 trembling motion. The mandibles form powerful crushing 

 lateral jaws. With regard to this jaw-apparatus, as compared 

 with that of the vertebrate, it should be noted (1) that it is 

 external to the mouth ; (2) that the jaws move from side to side, 

 and not up and down. 



The mouth itself is median and ventral, bounded at the sides 

 by the mandibles, in front by a shield-shaped plate or labrum, 

 and behind by the united bases of a pair of fleshy lobes which 

 form the metastoma. It leads into a short, wide oesophagus 

 lined with a thin chitinous coat continuous with the exoskeleton. 

 Salivary glands have been described as occurring in the walls of 

 the oesophagus, on the metastoma and first maxilliped, the 

 coxopodite of which is habitually tucked into the mouth. 



The stomach is large and sac-like, and is divided into a larger 

 anterior cardiac chamber and a smaller posterior pyloric chamber. 

 The chitinous lining of the oesophagus is continued into the 

 stomach, and in the dorsal region gives rise to a gastric mill 

 composed of more or less calcified ossicles. Posterior to this it 

 is folded inwards into the narrowed pyloric regions as valvular 

 projections beset with hairlike setae. This fold has been termed 

 the filter or strainer. 



