330 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



in the female a dark brownish green with a conspicuous 

 pinkish patch at each side at the origin of the pleopoda, 

 as in G. pulex. The male is usually a lighter yellowish 

 brown. The length of the adult male is about 20mm., 

 of the female 18mm. 



Spence Bate (1) remarks that the Amphipoda in 

 general do not throw off an injured limb, the wound 

 becoming cicatrized with a black scar. Gammarus pulex 

 appear able to withstand very low temperatures, though 

 they die instantly in warm water. Turton (23) and 

 Zenker (28) state that they are phosphorescent, but this 

 has iiot been confirmed by the writer in some casual 

 experiments. 



External Characters. 



The body is elongated and compressed laterally, with 

 the back evenly rounded. It is covered by a calcified 

 cuticle, which is thin enough to be partially transparent, 

 even in a full-grown specimen, and bears numerous spines 

 and setae. Externally, it consists of (1) the cephalo- 

 thorax, (2) the seven free segments of the thorax form- 

 ing the mesosome, (3) the first three segments of the 

 abdomen forming the metasome or pleon, and (4) the last 

 three segments of the abdomen forming the urosome or 

 urus, of which the last segment bears the telson. Thus 

 there are thirteen free segments in addition to the 

 cephalothorax and telson. Morphologically the body 

 is composed of nineteen segments and the telson ; of these 

 five form the head, eight are included in the thorax, and 

 six in the abdomen, of which the last bears the telson. 



The head is united with the first segment of the 

 thorax to form the cephalothorax, and is covered by a 

 eephalothoracic shield, produced downwards at the sides 

 into somewhat angular plates (PI. II.), which are 



