X PHYLUM ANNULATA 405 



Sub-Class I.— POLYCHiETA. 



Chsetopoda with the sexes distinct, and the ovaries and testes of 

 simple character and metamerically repeated. Highly developed 

 parapodia are present, in most instances, bearing numerous long 

 setae. There is usually a definite head with eyes and tentacles, 

 and often cirri and branchiae on the segments of the body. A 

 clitellum is never developed. A metamorphosis takes place: the 

 larva is a trochophore. Nearly all the Polychaeta are marine. 



Order 1. — Archi-Ch^top(3DA. 



Aberrant or primitive PolychtEta^ in which the nervous system 

 is not separated from the epidermis, and the ventral cord is not 

 segmented into ganglia. Only one genus {Sitccocimts). 



Order 2. — Phanerocephala. 



Polychteta with protriisible pharynx usually armed with chitinous 

 jaws. There is a well-developed head. The segments are 

 completely or nearly similar throughout the length of the body, 

 and the parapodia are usually equally developed throughout and 

 provided with cirri. The branchiae, when present, are not confined 

 to the anterior end. 



Order 3. — Cryptocephala. 



Polychseta devoid of protrusiblc pharynx and of jaws or teeth. 

 The head is frequently very small, and sometimes is devoid of 

 eyes or of tentacles, the prostomium sometimes much reduced and 

 co\-ered over by the peristomium. The body is distinguishable, by 

 differences in the form of the segments, parapodia, and seta^, 

 into two or even three regions. The parapodia are liftle 2Jrominent 

 in the posterior parts, and usually without cirri. The branchiae, 

 when present, are usually confined to the anterior end, and are 

 sometimes represented by modified cephalic palpi. 



Sub-Class II.-OLIGOCHiETA. 



Chsetopoda with the sexes united, the reproductive system com- 

 plicated, the ovaries and testes compact and never more than two 

 pairs of each. No definite paropodia are developed and no cirri, 

 and only a small number of simple setiE on each segment The 

 head is not distinct. A clitellum is usually present. There is no 

 metamorphosis. Mostly terrestrial or fresh-water forms. 



' The Ari:hi-0hii/iij)O(/(( are usually classed with the Polye/uela, but their 

 alliances are perhaps quite as close with the Oliyochata. In some respects 

 Sa(;cocirrus resembles Polygordius and Protodrilus (Arhci-Annelida (/. ('•) 

 but is distinguished from them by the possession of seta'. 



VOL. I H H 



