CHAETOPODA 143' 



groove of Frotodrilus has in this genus closed in, and forms a 

 canal within the nerve cord. Eed blood occurs in this genus, 

 and the dorsal vessel gives off lateral branches, which, however, 

 end caecally. The sexes are distinct, and the ovaries or testes 

 arise in the posterior segments (Fig. 90). 



The last member of the group, DinopMlus, is a minute 

 marine animal. Two species, B. gigas and D. taeniatus, have 

 recently been described by Weldon and Harmer from the coast 

 of Devonshire and Cornwall. The body consists of a head or 

 prostomium, which bears two eye-spots, and whose cilia are 

 uniform or arranged in two preoral circlets. The mouth opens 

 on the second segment or peristomium, and then follow five 

 or six segments, and finally a postanal unsegmented tail. In 

 both the above-mentioned specimens the entire ventral surface 

 of the animal is uniformly ciliated, and each segment has one 

 or two bands of cilia. D. vorticoides is uniformly ciliated all 

 over. The nervous system is in contact with the skin. The 

 coelom is traversed by strands of connective tissue in D. gigas ; 

 and in D. taeniatus there are more definite spaces connected 

 with the inner ends of the nephridia. In D. gigas an excretory, 

 system of the Platyhelminthine type, with flame cells, has 

 been described, but in D. taeniatus and D. gyrociliatus, 5 pairs 

 of nephridia are found, each with a triangular appendage 

 hanging into the lumen of their ciliated duct. The sexes are 

 separate, and in the male D. taeniatus the fifth pair of 

 nephridia appear to have become modified and form vesiculae 

 seminales. A penis is present, and seems to be inserted 

 indifferently into any part of the skin of the female. 



There is little doubt that Dinophilus should be classified 

 with the other members of the group Archiannelids ; on the 

 other hand, its median genital pore, the presence in some 

 species of the Platyhelminthine excretory system, and the 

 method of fertilisation adopted by D. taeniatus which is 

 paralleled in the Polycladida, support the view of the Platy- 

 helminthine origin of these worms. 



THE OLIGOCHAETA. 



Chaeacteristics. — The Oligochaeta are characterised hy the ab- 

 sence of antennae, parapodia, branchiae, and cirrhi. Their 



