DORSAL SHIELD. APPENDAGES. 437 



bearing groups, however, the base of the fold has extended 

 backwards and involved the terga of some or all of the segments 

 of the thorax, becoming in them a cephalothoracic shield. To 

 whatever degree the thoracic terga may be involved in the 

 shield its edges always project freely, investing the sides of 

 the thorax more or less closely, and the space thus enclosed may 

 be converted (Cumacea, Chelifera, Decapoda) into a respiratory 

 chamber. 



In Anaspides, Isopods and Amphipods a dorsal shield is absent, 

 and in the two latter orders the first thoracic segment (in the 

 Laemodipoda the first and second) is completely fused with the 

 head, forming a short cephalothorax. 



Behind the last appendage- bearing segment of the abdomen 

 there is in most Malacostraca a simple median plate, the 

 telson, with the anus opening on its ventral surface. In some 

 forms (A stacus), the telson is incompletely divided by a transverse 

 suture. In the Leptostraca two setose processes, jointed in the 

 larva, project backwards, one on either side of the anus, con- 

 stituting a caudal fork of the type found in the Phyllopods and 

 Copepods. This structure recurs in the larval stages of the 

 Mysidae, the protozoea stage of Penaeus, and in a larva which 

 has been referred to the Stomatopods (p. 509). 



Head appendages. Unlike those of the Entomostraca the first 

 antennae of the Malacostraca are frequently bi-, sometimes tri- 

 ramous in the adult, though uniramous in the larva. An oto- 

 cyst is present in the basal joint in Anaspides and in most Deca- 

 poda. The second antenna has a 2- or 3-segmented protopodite, 

 bearing a many- jointed flagellar endopodite, the three basal 

 segments of which are generally enlarged. The exopodite when 

 present usually consists in the adult of an oval or truncated 

 unsegmented scale, frequently fringed with setae. It is absent 

 in Cumacea, Amphipoda and most Isopoda. The antennary 

 (excretory) gland opens (except in Isopoda, where it is wanting) 

 on the ventral aspect of the proximal segment in cases where 

 the protopodite consists of two segments, on the second where 

 it consists of three. 



The basal segment of the mandible is produced inwards into 

 a prominent masticatory lobe, working against its fellow. The 

 shape of this lobe presents considerable modifications. It may 

 be simple, as in Astacus, but it is often produced into two pro- 



