404 INVERTBBBATE MORPHOLOGY. 



and is known as the telson. All these segments with the 

 exception of the telson, and in the Leptostraca of the seg- 

 ment immediately in front of it, bear appendages. Folds of 

 the integument forming a cephalothoracic carapace are fre- 

 quently present, but it is rare that a bivalved shell occurs. 



The stomach is always provided with chitinous teeth and 

 forms an efficient masticatory organ, and lateral eyes are 

 present except in some Cumacea and in some forms belong- 

 ing to other groups which inhabit caves or the depths of the 

 ocean, under which conditions the eyes become rudimentary. 

 The openings of the female reproductive organs are always 

 situated on the basal joints of the appendages of the sixth 

 thoracic segment, and the male openings on the appendages 

 of the eighth segment. The antennary gland is usually well 

 developed, while the shell-gland is either rudimentary or 

 wanting in the adult. 



Although numerous rather small forms belong to this 

 class, yet on the whole they much surpass in size the Ento- 

 mostraca, some forms even reaching a length of over 50 cm. 

 A few forms, such as Ewphausia and Fencetis, leave the egg as-a 

 Nauplius, but in the majority this stage is passed before 

 hatching, the embryo first leading a free existence at a later 

 stage in the larval form known as the Zoea, though in some 

 cases hatching may be retarded until later stages, in fact 

 sometimes until the adult form is acquired. 



I. Subclass Leptosteaca. 



The Leptostraca are exceedingly interesting forms, present- 

 ing similarities to the Entomostraca on the one hand and to 

 the Malacostraca on the other, thus connecting the two 

 classes. They are exclusively marine in habitat and possess 

 a thin bivalved shell- duplicature which is provided with an 

 adductor muscle and is prolonged in front into an unpaired 

 plate which covers the dorsal surface of the head. 



The antennules (Fig. 183, at') consist of a three-jointed 

 basal portion bearing in addition to the multiarticulate flagel- 

 lum a scalelike exopodite, a structure wanting in the antennse 

 (af), which otherwise have a similar form. The mandibles 



