334 THE INVERTEBRATA 



of some or all of the thoracic somites (the Cladocera, most of the 

 Malacostraca) : such somites are not on that account alone to be 

 regarded as included in the head, though they may become so. The 

 chamber enclosed by the carapace is known in various cases by various 

 names as gill chamber, mantle cavity, etc., and performs important 

 functions in sheltering gills or embryos, directing currents of water 

 which subserve feeding or respiration, etc. In front, the carapace is 

 continuous with the dorsal plate which represents the terga of the 

 head, the cervical groove, if present, marking the boundary between 

 them. We shall apply the term dorsal shield to the structure composed 

 of the dorsal plate of the head with the carapace, if the latter be 

 present.^ The dorsal plate of the head may be prolonged in front as 

 a projection which is called the rostrum (Fig. 283, rs). 



A glandular patch or patches on the dorsal surface of the head, near 

 its hinder limit, in many of the Branchiopoda, in Anaspides, and in the 

 young stages of various other crustaceans, is known as the dorsal 

 organ or neck gland. It is used by cladocera and conchostraca for 

 temporary fixation. In other cases its function is not known. Possibly 

 the organs to which this name is given are not all homologous. They 

 must not be confused with the "neck organ" of branchiopods (see 

 p. 342). 



Of the appendages or limbs of the Crustacea, the first, or antennule, 

 is a structure sui generis, not comparable in detail with any of the 

 others. Typically it is uniramous, and though in many of the Mala- 

 costraca it has two rami, these are probably not homologous with the 

 rami, described below, of other appendages. The remaining limbs 

 may all be reduced to one or other of two types — the "biramous" 

 limb usually so-called, to which most of them more or less clearly 

 conform, and the phyllopodium, to which belong the trunk limbs of 

 the Branchiopoda and some other appendages, chiefly maxillules and 

 maxillae and notably the maxilla of the Decapoda. The name by 

 which the first of these types is generally known refers to the fact that 

 limbs which best represent it fork distally into two rami. Since, how- 

 ever, the phyllopodium possesses the same two rami, and bears them, 

 though not as a distal fork, yet in the same way as a great number of 

 limbs of the first type, it is well not to use a name which might imply 

 that there is a constant difference in respect of the rami between the 

 limbs of the two types. We shall therefore called the first type the 

 stenopodiiim, referring to its usually slender form (Gk. orevos, 

 narrow). 



^ These terms have been used in various senses. In the usage here pro- 

 posed, when there is no carapace fold the dorsal shield is the dorsal plate 

 of the head together with the terga of the somites (if any) that are fused with 

 the head. 



