Characteristics of Crustacea. cxix 



and the subordination of these thoracic segments to the cephalic, 

 may be still further manifested by a fusion of the seg-ments them- 

 selves into a cephalo-thoracic carapace. The post-thoracic segments 

 in Crustacea are more obviously divisible into an abdominal and 

 post-abdominal series, than they are in such other Arthropoda as 

 do possess a post-abdomen. The normal number of the abdominal 

 segments is five, and the appendages they carry are in the adult 

 state very ordinarily either alone, or in association with the post- 

 abdominal segments and appendages, the locomotor organs. Ac- 

 cordingly as only one of the three pairs of thoracic appendages, or 

 as all three lose their primordial and typical locomotor functions 

 and become associated with the oral appendages, the higher Crus- 

 tacea are classified as ' Tetradecapoda ' (Dana) (the five abdominal 

 segments furnishing ten, and the two posterior thoracic four limbs), 

 or as ' Decapoda.' The normal number of the caudal or post- 

 abdominal segments is six, the appendages of the first four of 

 which may be modified so as to carry ova, or branchiae, but do not 

 attain the importance as locomotor limbs, which those of the five 

 abdominal segments do. 



The appendages of the sixth segment, however, very ordinarily 

 attain considerable importance as locomotor organs, and they con- 

 stitute, together with a median azygos element, developed poste- 

 riorly to the sixth segment, the powerful ' swimmeret' of the Macru- 

 rous Decapoda. Their morphological importance is equally great, as 

 they correspond with the caudal feet of naupliiform larvae, and 

 together with the two pairs of antennae which they may resemble 

 by carrying, as in Mi/sis, organs of special sense, and the mandibles, 

 make up the entire sum of the appendages of the ' primitive body ' 

 of Crustacea, as represented by those forms. The bilateral append- 

 ages which certain lower Crustacea carry at the posterior extremity 

 of their bodies, are not articulated ventrally as these caudal feet are, 

 and on this account, as also upon others, they cannot be considered 

 as homologous with them. There is much difference of opinion as 

 to whether the mesially-placed azygos element of the swimmeret 

 ought to be counted as a seventh post-abdominal segment. (See 

 p. 11^, infra.) 



The mandible is ordinarily provided with a palp, which in a 

 Class with but few exceptions aquatic in habit, serves to direct 

 floating food towards the mouth. There are no salivary glands in 



