330 



Introduction to Animal Morphology. 



Fig- 34- 



raly pieces, while the ventral consists of two central 

 sternal and two episternal portions. These plates are 

 always united, and at their sutures send in apodemata 

 or processes. In lower forms the number of somites 

 is varied by suppression or multiplication (Apus). 

 The normal number of post-abdominal somites is six, 

 of abdominal, five, of thoracic, three, and of cephalic, 

 six. 



The chitinous layers are usually calcified ; the 

 cuticle* often contains unicellular glands opening into 

 the pore canals, sometimes clustered (Argulus), and 

 stellate pigment cells, green, purple and red — the 

 latter colour being the most permanent, as boiling or 



reagents redden the others. 



The limbs vary in each class, but 



in the higher forms the arrangement is 



as follows : — Pre-orally there are two 



pair of antennae, the anterior smaller 



(antennules), and the hinder larger : 



the former usually consist of three ba- 

 sal joints (thebasi-, ischio-, and mero- 



podite), terminated by a many-jointed 



annulated feeler or pair of feelers : 



the latter has five basal joints (coxo-, 



basi-, ischio-, mero-, and carpo- 



podite), and a long, ringed, feeler, 



like the foregoing. The sternum of 



the antennary somite is called episto- 



ma, and behind it is t\ieclypeuSy labium, 



or upper lip. The post-antennar seg- 

 ment bears as its appendages on each 





Hippol)'te Gordoniana ; 

 b, external antenna ; d, 

 inner antenna ; e, telson. 



* In Sapphirina the cuticle is marked on its surface by a system of 

 wavy lines which give it an irridescent appearance. 



