//. TYPES IN DETAIL WITH THEIR SUBORDINATE GROUPS. 21 



Class I. CHJETOPODA Typical Annelids, with well- 

 marked external somites, cor- 

 responding to the internal meta- 

 merism; somites furnished with 

 paired groups of chitinous bristles. 



Order i. Polych;eta Bristles conspicuous, situated on 



raised lateral portions, the para- 

 podia. Head generally present. 



X Nereis, Serpula. 



Order 2. Oligoch^eta Bristles very small, sunk in hol- 

 lows along the sides; no parapodia ; 

 no distinct head; hermaphroditic. 

 X Nais, Liimbricus. 



Class II. HIRUDINEA Aquatic hermaphroditic ecto-para- 



sites; segments without bristles; 

 move by terminal, adhesive suck- 

 ers. External and internal seg- 

 ments do not correspond. 



Order i. Rhynchobdellid^. ...Pharynx extensile, forming a sort 



of proboscis. x Clepsine. 



Order 2. Gnathobdkllid^ . . . .Pharynx not extensile, with three 



longitudinal ridges which are often 

 toothed. X Hirudo. 



SUPPLEMENTARY CLASSES (PROBABLY RELATED TO ANNELIDS). 



Class CHjETIFERA, formerly taken with Sipunculoidea to form Class 

 Gephyrea. Here belong a very few marine worms, Echiurus, Bonellia, 

 etc., which show affinities to the Chaetopoda. They are segmented only 

 as larvae, but possess paired nephridia and a system of blood-vessels 

 similar to that of Annelids. 



Class Sipunculoidea includes a very few forms of marine worms, 

 bearing some slight affinity to Annelids. They were formerly united 

 with Chsetifera to form the class Gephyrea. They are cylindrical, unseg- 

 mented forms without bristles, and possess a retractile proboscis. 



X Sipunculus, Phascolosoma. 



Class CHjETOGNATHA. This includes one form, Sagitta, a small, 

 transparent, unsegmented worm found on the surface of the ocean. The 

 body is flat and possesses lateral fin-like extensions. The mouth is 

 armed with jaws beset with sharp hooks. Hermaphroditic. It develops 

 lateral coelom sacks, which appear to have the value of a metacoelom. 

 It is placed by some with the Nematodes. [Cf. p. 14.] 



