CRUSTACEA 



347 



Order ASCOTHORACICA 



Parasitic cirripedia, which have an alimentary canal from which 



diverticula extend into the mantle ; 



six pairs of thoracic appendages; 



and a segmented or unsegmented 



abdomen ; and are not attached by 



the preoral region. 



These animals are parasitic and 

 often imbedded in the tissues of 

 their hosts. They are an early 

 branch of the cirripede stock which 

 has retained the abdomen, in 

 some cases well segmented and 

 provided with movable caudal rami, 

 and has not the characteristic 

 mode of fixation by the antennules, 

 or frontal horns in the Nauplius. 



Laura, imbedded in the tissues 

 of the antipatharian Gerardia, has 

 the mantle in the form of a very 

 spacious sac with a narrow opening. 



Synagoga (Fig. 252), external Fig. 252. Sj'na^o^a Tw/ra. After Nor- 

 parasite on Antipathes, has a bi- "^^^- ^?-^' ^'^^ abdominal somite; 

 ^ , 1 r 1-1 11 ^"- antemiule; car. carapace; M. 



valve mantle, from which usually ^outh; ram. ramus of caudal fork; 

 protrudes the long abdomen of ^e/.telson; i/?. thoracic limbs, 

 four segments and a telson. 



Class MALACOSTRACA 



Crustacea with compound eyes, which in typical members of the 

 group are stalked ; typically a carapace which covers the thorax ; the 

 mandibular palp, if present, uniramous; a thorax of eight somites 

 and abdomen of six (rarely seven), all (except the 7th abdominal) 

 bearing appendages; and a complex proventriculus. 



The Malacostraca contain a very large number of species, which 

 exhibit great diversity. Nevertheless they are capable of reference to 

 a common type in respect of more features than the members of any 

 other group, though the Copepoda approach them in this. The ideal 

 malacostracan has twenty somites, including the preantennulary and 

 excluding the telson. Of these, six belong to the head (p. 296), eight 

 constitute the thorax, and six the abdomen. This number is only 

 departed from in the Leptostraca, which have an additional somite 

 at the end of the abdomen. (In the embryos of Mysidacea such an 



