GAMMARTJS. 339 



covering for the body, and penetrates deeply between the 

 joints of the segments and legs. It is formed by the 

 cellular hypodermis, which underlies it, and is a typical 

 cylindrical epithelium, with large, deeply-staining 

 nuclei. Masses of adipose tissue, formed of large, 

 rounded, non-staining cells, without intercellular sub- 

 stance, ensheathe the alimentary canal and gonads, filling 

 up the space between the former and the heart. This 

 adipose connective tissue is also found attached to the 

 ventral nerve cord, and separating the bands of muscle. 

 It connects the various organs with one another and the 

 body-wall, and is believed by Wrzeniowski (27) to 

 form the serous covering of the heart. A thin plate 

 attached to the base of the heart and the sides of the body 

 forms the pericardium (PI. IV., fig. 1, p.), and divides 

 the body cavity into an upper half, the pericardial cavity, 

 and a lower one, the sternal sinus (p.c. and st. s.). 



The body-cavity is a system of blood lacunae, as in 

 Astacus, the alimentary canal and its diverticula, as well 

 as the nervous system, being washed by the colourless 

 blood filling the sternal sinus, which has no definite walls. 

 The end-sacs of the nephridia at the base of the second 

 pair of antennae (PI. II., e.s.) are, probably, the only 

 remnants of the true coelom, as in Pal (em on serratus, 

 according to Weldon (26). 



The Digestive System. 



The slit-like mouth (PI. II., m.), which opens out of 

 the deep sinus between the two lateral extensions of the 

 cephalothorax, leads into a short, wide oesophagus con- 

 nected by bands of muscle surrounding it and united with 

 the exoskeleton. The opening of the oesophagus into the 

 ventral wall of the stomach is guarded by two pairs of 



