1907.] NATURAL SCIENCES OF rillLADELrillA. 113 



with the muscle recalls the precisely similar relation found between 

 the longitudinal nerves and muscles found by Meyer (1901) to exist 

 in the trochophore larva of Lopadorhynchus. and strongly hints that, 

 as in tliat form, it may l)e due to a common origin from the ectoderm, 



4. Alimcntanj Canal. 



The alimentary canal of D. conkUni does not differ in its essential 

 features from that described for other members of the group, and 

 resembles very closely that of D. apatris and D. gyrocUiatus. On the 

 ventral side of the head is the mouth opening, which is triradiate or 

 Y-shaped . the vertical limb of the Y being directed caudad. The latter 

 is not confined to the head segment alone, but extends backward into- 

 the first segment of the trunk as far as its middle region (see text fig. I 

 and fig. 2.3). Korschelt (1S82) has described a similar condition in D. 

 apatris, the mouth in this species extending back from the head into^ 

 the short segment-like piece which lies between the head and the first 

 trunk segment. In Schmidt's drawing (1856) of D. gyrocUiatus the 

 mouth is represented as confined exclusively to the head, but in another 

 of the same species published by Meyer (1886) the relation of the 

 mouth to the head and trunk is shown to be the same as that just 

 described for D. conklini. The possible significance of this relation 

 will be discussed later. 



The mouth leads dorsad directly into the oesophagus; caudad it 

 opens into the cavity containing the proboscis (fig. 23), an organ com- 

 mon to all or nearly all of the members of this group. Forming the 

 dorsal wall of the proboscis cavity and separating it from the oesophagus 

 is a crescentic fold extending across the posterior wall of the oesophagus 

 and partially closing its lumen; the two lateral halves of this are called 

 by Harmer (1889a) "the inner lips" (figs. 4 and 23, i.l.), in contra- 

 distinction to "the outer lips" {o.l.) which are formed by the union of 

 the walls of the body and oesophagus. From the inner lips the oesopha- 

 gus ascends, curving slightly backward, to the dorsal body wall, where 

 it bends sharply to the rear and, pursuing a horizontal course for a 

 short distance, reaches the stomach. This it enters at a point consider- 

 ably dorsad of the mid-level. The lumen of the oesophagus in its 

 ascending portion is scarcely more than a cleft, being very narrow 

 in its transverse diameter, although long in an antero-posterior direc- 

 tion. In its anterior portion, however, next to the wall of the esopha- 

 gus, the lumen widens out suddenly, forming here a small open channel. 

 The narrow portion of the oesophageal lumen answers to that limb or 

 radius of the Y-shaped mouth which is directed caudad, the open 



