212 



Arthropoda. Class 1. Crustacea. 



with a swimming ramus, the exopod, but only the first pair possess 

 epipods. 



The first pair of thoracic appendages is modified to form 

 maxillipeds; the second pair is also different from the rest. The 

 abdominal appendages, with the exception of the last pair (those of 

 the caudal fin) are, in the females always, in the males often, feebly 



Fig. 173. Boreomysis megalops, one of the Mysidse, $ . 1, 6 first and sixth abdomina 

 segments ; A^ — A^ first and second antennee ; ex exopod of the last thoracic appendage ; 

 ffj fifth abdominal appendage ; K^, Kj thoracic appendages, md mandibular palp ; Ot Otolith, 

 R brood sac, S carapace.— After Sars. 



developed. An auditory vesicle is found in the inner ramus 

 of the last pair of abdominal appendages, it is furnished internally 

 with a number of hairs supporting a large otolith (the Mysidse 

 are the only Crustacea possessing an auditory organ in such a 

 position). The inner, membranous side of the carapace is provided 

 with a close vascular network, and acts as a 

 respiratory organ. The epipod of the first 

 thoracic limb is situated within the branchial 

 cavity, and its movements cause a constant current 

 of water through the chamber. Prom the inner 

 sides of the basal joints of some of the thoracic 

 appendages arise thin, curved lamellse, which 

 together form a ventral brood-sac (marsupium) 

 to serve for the protection of the eggs and larvee. 

 The young ones leave the eggs as nauplii with 

 the three usual pairs of appendages (antennae and 

 mandibles), but are incapable of free movement; 

 they feed on the food yolk derived from the egg, 

 and only leave the brood-pouch when they have 

 acquired the general appearance of the adult. 



Some Mysidas are found in the open sea, 

 others are littoral : the genus Mysis, for example, 

 lives on the coasts of Northern Europe; it is a transparent feebly 

 pigmented animal occurring in shoals. 



Fig, 174. Mysis- 

 nauplius, seen 

 from below (en- 

 larged), ctj, o^ first 

 and second antennsB, 

 md mandible. — Orig. 



