132 PROCEEDINGS OF THE ACADEMY OF [Feb., 



but with an important exception: in Dinophilus it is apparently not 

 manifested in the sex organs, the homologues of those organs and tissues 

 which are, in the annelids, the products of the mesoderm bands. 



The number of segments, as indicated by the body wall in its external 

 divisions and ciliated rings, does not agree with that indicated by the 

 other metamerically arranged parts. The former — the trunk segments 

 as delimited by constrictions and their ciliated bands — indicate six 

 trunk segments, while the latter, namely, the mucous glands, the nerve 

 ganglia and the nephridia, testify to only five. Examination of text 

 fig. Ill and fig. 1 will show that the segment not represented by ganglia 

 or mucous glands is the first postoral. It may be added that this 

 segment is also not represented by the nephridia, since, although the 

 first pair of nephridia do actually extend into this segment, the greater 

 part of these organs lie in the segment following, and obviously belong 

 to the latter. That division of the trunk, then, immediately following 

 the neck and bearing the first band of cilia is evidently not a true 

 metamere, but is to be interpreted as a minor annulus, a subdivision 

 of a metamere having occurred, as in all leeches and in some Oligochccta 

 and Polychcvta. The next succeeding annulus, the second, is clearly 

 the corresponding major annulus, and the two together compose the 

 first trunk metamere. The development of an additional annulus in 

 this region is probably to be traced to the demand for greater room on 

 the part of those structures which lie within the first trunk metamere, 

 namely, the proboscis, a relatively large organ, the first pair of ne- 

 phridia and the first pair of trunk ganglia; the two latter much exceed- 

 ing in size those of the succeeding segments. 



Five trunk metameres are also present in D. tccniatus (Harmer, 1889a) 

 and in D. vorticoides (Schimkewitsch, 1895), as indicated by the num- 

 ber of the trunk ganglia and of the nephridia. Meyer (1886) shows 

 five pairs of nephridia also in his figure of D. gyrociliatus; the number 

 of trunk ganglia, is, however, unknown, since the nervous system of 

 this species has so far not been carefully investigated. This species 

 (gyrociliatus), as judged by Meyer's figure, displays a condition pre- 

 cisely similar to that obtaining in D. conklini, since there apperas to 

 be present six trunk segments, in the second of which the first pair of 

 nephridia are situated. The first trunk metamere is therefore evi- 

 dently also composed of two annuli, of which the posterior is the major. 

 In D. vorticoides (Schimkewitsch, 1895) one of the trunk metameres 

 appears also to have been subdivided into annuli, since the trunk bears 

 twelve double rings of cilia. As to the particular metamere which has 

 .subdivided, it is impossible to conclude from Schimkewitsch's figures, 



