470 CRUSTACEA MALACOSTRACA. 



of the first antennae, a peculiar tuft of hairs being present at 

 the end of the shaft. 



Development. The eggs of the Hemitropha hatch out, as 

 shown by Metschnikoff (Euphausia),in the nauplius stage, having 

 the usual character of the appendages, but with the eye and 

 anus as yet undeveloped. In the course of their metamorphosis 

 they pass through metanauplius, protozoaea and zoaea stages 

 (Fig. 291), the latter (described by Dana as Calyptopis) differ- 

 ing from the Decapod zoaea in possessing only one pair of 

 maxillipeds. The margins of the shield of the larva are curiously 

 serrated. Though the segments are formed in order from be- 

 fore backwards, the abdominal appendages begin to appear 

 before the last thoracic, and their full number is completed first. 



In the development of Mysis, which occurs in the brood 

 pouch, the larva hatches in a stage corresponding to the nauplius. 

 The nauplius skin is retained as an envelope within which the 

 metamorphosis progresses. All the thoracic limbs are formed 

 simultaneously. A caudal fork is present in the larva, as is 

 usual in the Entomostraca. 



All known Schizopods are inhabitants of salt-water except 

 Mysis relicta, widely distributed in fresh- water lakes in Europe 

 and N. America, and a few other Mysidae, which also live in 

 fresh- water. Several forms allied to Mysis are indigenous to 

 the brackish water of the Caspian. 



Many are pelagic and often congregate near the surface of the 

 sea in enormous numbers. The shoals of Euphausia inermis 

 form the chief constituent of the food of the hump-back and 

 other whales, which follow them in their movements during the 

 summer months, accompanied by hosts of fish and birds. A 

 species of Mysis furnishes food for the herring. They are 

 especially abundant in arctic seas, but are widely distributed 

 over the globe. Gnathophausia, many species of which were 

 obtained in the voyage of the Challenger, inhabits deep water. 

 A discussion of the systematic relations of the Schizopoda is 

 given above (p. 453). 



Tribe 1. HEMITROPHA. 



First resemble the succeeding thoracic legs. Posterior part of dorsal 

 shield not free from the underlying segments. Mandible without a lacinia 

 mobilis. Heart short, polygonal. The eggs carried in ovisacs, and the 

 young hatched as nauplii. 



