218 



INVERTEBRATE MORPHOLOGY. 



0(1 



Trochophore are related to each other, but hardly with so close au affinity 

 as would be implied by a statement that they are persistent Trochophores. 



II. Subclass OLIGOCH.ETA. 



The OKgochaeta are with few exceptions fresh-water or ter- 

 restrial Chaetopods, arid present a much simpler body form 



than do the Polychaets. The first 

 segment or prostomiiDii (Fig. 103, 

 pr} is devoid of tentacles or cirri, 

 and only in a few forms are eyes 

 present upon it. The body is divided 

 into well-marked segments, but 

 parapodia are lacking, though in 

 the majority of forms setse. arranged 

 in a more dorsal (s') and a more 

 ventral (s) group, occur on the sides 

 of each metamere ; in a few forms, 

 however, a single series of groups 

 only is present, while in Perichceta 

 the setae are arranged in a ring 

 around each metamere, and in Ana- 

 chceta they are wanting, their place 

 being indicated only by the sacks 

 in which in other forms they are 



FIG. 103,-ANTERioKENDOF c l eve loped and which project into 



r.iiiu/it'jr u v 



the coelom as large hypodermal 



glands. As a rule, too, no branchiae 

 are present, the blood being aerated 

 through the walls of the body, mi- 

 nute branches of the blood-vessels 

 penetrating into the hypodermis in 

 vd = opening of vas defereus. the terrestrial forms ; a few aberrant 



forms, however, possess either dorsal 



or ventral (Clmtobrancltus) appendages, which are probably 

 respiratory in function, on many of the segments, while in 

 the genus Dero the finger-like processes of the terminal 

 metamere are probably branchiae. 



The exterior of the body is covered by a well-marked cuti- 

 cle, and beneath it lies the ectoderm or hypodermis, usually 



cus. 



C = clitellmn. 

 gl = glands. 



m = mouth. 



od = opening of oviduct. 



pr = prostomium. 



s = ventral setae. 



s' = lateral setae. 



