130 ELEMENTARY BIOLOGY. [CHAP. 



face to which they are attached, the three pairs of append- 

 ages 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 (ophthalmites\ 

 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 ; 

 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 appendages (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. In correspondence with the number of 

 appendages the body consists of twenty somites; of which six 

 remain moveable upon one another to form the abdomen, 

 while the other fourteen are united to form the cephalothorax. 



The branchiostegite is an outgrowth of the dorsolateral 

 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 

 praeoral and post -oral somites. 



Thus the whole skeleton in these animals may be con- 

 sidered as a twentyfold repetition of the ring-like somite 

 with its pair of appendages, which is seen in its simplest 

 form in one of the abdominal somites. Moreover, not- 

 withstanding the great variety of functions allotted to the 

 various appendages, the study of the details of their struc- 

 ture (see Laboratory work) will shew that they are all re- 



