DORSAL SHIELD. APPENDAGES. 343 



but in many cases fusion has occurred between the head and 

 one or more of the anterior thoracic segments to form a eephalo- 

 thorax. This fusion may depend simply on the obliteration of 

 the grooves between the segments, as in Copepoda and the 

 Isopods and Amphipods, but it is frequently associated with 

 the development of a structure characteristic of many groups 

 of Crustacea, the dorsal shield. This is primarily a reduplica- 

 ture of the integument of the head, extending backwards over 

 the body, and more or less completely envelqping it. In the 

 Cladocera (Fig. 252) it forms an incomplete bivalve shell 

 enveloping the hinder part of the body, but the head is not 

 enclosed by it. In the Ostracods (Fig. 253) and some Branchio- 

 pods the whole of the body can be enclosed between the valves 

 of the shell, and shut up within it by the action of an adductor 

 muscle (Fig. 253, 13} passing between them. 



Although the dorsal shield is primarily a fold of the integu- 

 ment of the head, the base of the fold may extend backwards 

 (as in Decapods) involving some or all of the terga of the thoracic 

 segments, and in this manner emphasizing the fusion of the two 

 regions in the cephalothorax. 



The terms carapace and shell have been used in different senses. They 

 may imply the fused terga of the head and of a variable number of tho- 

 racic segments ; but are sometimes synonymous with " dorsal shield." 



The fold contains blood sinuses, and may, when the shell is 

 thin, form an important respiratory organ. In the Entomo- 

 straca the coils of the excretory organ, hence known as the shell 

 gland, are included in it (Fig. 252, Sd). In the Cirripedes, as 

 will be explained more fully under this group, a large part of 

 the tissues of the head become, in the adult, included in the fold. 



An upper lip (hypostoma) and lower lip are generally present, 

 in front of and behind the mouth. The latter may be con- 

 spicuously bilobed, but it is always distinct from the appen- 

 dages in the Crustacea. 



Appendages. With the exception of the first antennae the 

 appendages throughout the body, and in the several groups, 

 though modified to subserve the most varied functions, conform 

 in a remarkable manner to a common type the biramous limb' 

 of the Crustacea. This consists of a shaft or stem the proto- 

 podite, usually regarded as composed of two segments, the 

 proximal or coxopodite, articulating with the segment of the 



