168 OLIGOCHAETA 



which traces exist in Aeolosoma ; and (3) the development of the clitellum (in 

 Mesenchytraeus at any rate) laterally and ventrally, but not dorsally ; (4) origin of 

 dorsal vessel from a perienteric sinus. 



Together with these resemblances to other groups of Oligochaeta, the Enchytraeidae 

 are to be distinguished by certain peculiarities restricted to themselves. These are : 

 (i) the peculiar form of the nephridia 1 ; (2) the generally elongated and glandular 

 funnel of the sperm-duct ; (3) the reduced size and simple structure of the spermiducal 

 glands ; (4) the frequent presence of a double layer of longitudinal muscles. 



These four classes of facts in the organization of the Enchytraeidae appear to me 

 to refer them to an isolated position between the two stems of origin of the Oligochaeta. 



The Enchytraeidae perhaps resemble Phreoryctes more than any other group of the 

 higher Oligochaeta ; these resemblances, however, are not numerous, and are confined 

 to a few species. The most striking is the existence in various species of Pachydrilus 

 of the segmentally arranged lateral outgrowths of the nerve-cord ; structures similar to 

 these appear to occur in Phreoryctes. Besides Phreoryctes, the only Oligochaet in which 

 there are four setae per segment, implanted singly, is Enchytraeus monochuetos. 



The next point to be considered is the connexion between the Microdrili, the 

 Megadrili, and the Aphaneura. In attempting to frame the broad outlines of a phylo- 

 genetic scheme, such as is implied in the question here raised, regard must be taken 

 of the relationships of the Oligochaeta as a whole to other groups of animals. 

 A proper phylogenetic scheme can only be elaborated after considering these relations. 



The Oligochaeta are almost universally regarded as nearly akin to the Polychaeta ; 

 the two groups are in fact generally placed within one larger group called the 

 Chaetopoda. In so placing them it appears to me that important differences have 

 been lost sight of, and too much stress has been laid upon the broader points 

 of resemblance. What are the chief likenesses between the Polychaeta and the 

 Oligochaeta? They are both regularly segmented worms, with a well -developed 

 coelom, also arranged in accordance with the segmentation ; each segment has 

 typically a pair of nephridia, which may in both cases be secondarily increased in 

 number ; they have chitinous setae derived from the epidermis ; in both there is a 

 closed vascular system, much complicated, and usually containing red blood, coloured 

 so by haemoglobin ; in both the gonads are proliferations of the coelomic epithelium, 

 and are persistently unenclosed in special cavities, except in a few cases ; the ducts 

 for the carrying away of the genital products are formed out of metamorphosed 

 nephridia; lastly, the nervous system is constructed on a similar plan. 



1 I do not refer so much to the network formed by the lumen, which is to be found elsewhere, e.g. 

 Chaetogaster, as to the general form of the organ. 



