406 INVERTEBRATE MORPHOLOGY. 



II. SUBCLASS THORACOSTRACA. 



The Thoracostraca are characterized by the occurrence 

 throughout the group of a well-developed duplicature of the 

 body-wall, arising from the posterior head-segments and 

 covering in a greater or less number of the thoracic segments, 

 constituting what is termed a carapace. On the dorsal sur- 

 face it fuses with the body-wall, but, at the sides encloses a 

 respiratory chamber in which the branchiae, when present, lie. 

 According as the carapace extends over all or ouly over the 

 anterior thoracic segments a more or less perfect cephalo- 

 thorax is formed, a fusion of the covered thoracic segments 

 with each other and with the head-segments occurring, the 

 abdominal segments remaining in all cases distinct. 



Branchiae, consisting of bunches of hollow thin-walled 

 processes whose cavities communicate with the lacunar spaces 

 of the body, are borne by certain of the appendages except in 

 the Mysidese. The lateral eyes except in the Cumacea are 

 stalked and the antennary gland is usually well developed. 



1. Order Schizopoda. 



The carapace in the Schizopoda covers in the entire 

 thorax, but a certain number of the posterior thoracic seg- 

 ments remain ununited with it. The antenuules are bira- 

 rnous, as are also the antennae (Fig. 184), the exopodite in the 

 latter case being represented by a scalelike structure. The 

 thoracic appendages are all similar and are biramous, the 

 endopodites being limblike structures tipped by claws, 

 while the exopodites are multiarticulate Hagella. In the 

 genus Eupliausia the two last pairs are quite rudimentary, 

 their branchiae remaining, however, well developed. The two 

 anterior pairs in the genus Mysis have their basal joints en- 

 larged to form jaws and consequently are distinguished as 

 maxillipeds, but in Euphausia this distinction does not occur. 

 The abdominal appendages in the females are generally small 

 with the exception of the sixth pair, and in the genus Mysis 

 are quite rudimentary. In the males of all genera they are, 

 however, well-developed biramous swimming-feet, and the 

 sixth pair in both sexes forms with the telson a tail-fin. 



