I?6 ELEMENTARY BIOLOGY. [CHAP. 



is the cephalic flexure. In correspondence with this change of 

 position of the surface to which they are attached, the three 

 pairs of appendages of the somites which lie in front of the 

 mouth are directed either forwards, or forwards and upwards. 

 The posterior pair consists of the long feelers or antenna: the 

 next, of the short feelers or antennules ; and the most anterior 

 is formed by the short subcylindrical stalks (pphthalmites], 

 on the ends of which the eyes are situated. 



This enumeration shews that the Lobster and Crayfish 

 have six pairs of abdominal appendages the swimmerets 

 and " tail-fin"; eight pairs of thoracic appendages (four pairs 

 of ambulatory limbs, one pair of chelate prehensile limbs, 

 three pairs of maxillipeds), and six pairs of cephalic ap- 

 pendages (two pairs of maxillae, one pair of mandibles, 

 one pair of antennae, one pair of antennules, one pair of 

 eyestalks), making in all twenty pairs of appendages. It 

 may or may not be that the eyestalks are modified ap- 

 pendages ; should they have that value, the body in 

 correspondence with the number of appendages will con- 

 sist of twenty somites (the telson excepted): of these six 

 remain moveable upon one another to form the abdomen, 

 while the other fourteen are, with the exception of a portion 

 of the last thoracic one in the Crayfish completely united 

 to form the cephalothorax. 



The branchiostegite is an outgrowth of the dorso-lateral 

 region of the confluent thoracic somites. The serrated 

 rostrum which ends the carapace is a fixed median pro- 

 longation of the dorsal wall of the anterior cephalic somites; 

 while the telson is a moveable median prolongation of the 

 dorsal wall of the sixth abdominal somite. The labrum and 

 the metastoma are median growths of the sterna of the 

 prae-oral and post-oral somites. 



Thus the whole skeleton in these animals may be con- 



