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which it issues to the outside. Connected by a peduncle 

 with its absorptive " roots," it becomes an ovoid growth 

 partly concealed by the crab's abdomen. It is now an 

 adult " Sacculina externa " and consists of a central " vis- 

 ceral mass " which contains hermaphrodite organs and 

 ducts, cement glands, and a nerve ganglion, and which 

 is surrounded by a brood-chamber that opens to the ex- 

 terior. 



Astacus (The Fresh-water Crayfish). 

 An Example of the Malacostraca. 



Describe the External Features of Astacus. 



The head and thorax form a rigid cephalothorax covered 

 by the carapace, a saddle-like shield with a frontal spine 

 (the rostrum) at the sides of which are the eyes, on movable 

 stalks. The abdomen has six movable segments and a 

 flat tailpiece or telson, which, with the last pair of append- 

 ages, forms the paddle used for swimming backwards. 



The transverse cervical or neck groove on the carapace 

 marks the junction of head and thorax. The dorsal por- 

 tions of the cephalothorax segments have been fused in 

 the carapace ; but ventrally the segmentation is distinct. 

 The floor of the cephalothorax is double ; the inner frame- 

 work, the " endophragmal skeleton," is formed by folded 

 ingrowths of cuticle (apodemes) to which the limb muscles 

 are attached. The sides of the carapace shelter the gills 

 on the thoracic walls and thus form branchial plates 

 (branchiostegites). 



The exoskeleton of an abdominal segment consists of 

 four parts : a dorsal arched tergum with side extensions 

 (pleura), a ventral crosspiece or sternum and, between each 

 pleuron "and the sternum, a small epimeron, at the inner 

 side of which an appendage is attached. The abdomen can 

 be flexed and extended : each segment slightly overlaps 

 the one behind it, and has two little knobs which fit 

 into corresponding sockets on the overlapped segment. 



There are nineteen pairs of (serially homologous) append- 

 ages,* one pair to each segment. The mouth is on the 

 ventral surface of the head ; the anus is ventral on the 

 telson. 



