SOLBNOGASTRES. 3 



shaped sense-organ is wanting, though usually present in other species. The digestive 

 tract opens to the exterior at the same place as the mouth-cavity, with which it has, 

 however, no communication [cf. Dinomenia, 6, p. 12, PI. 2, Fig. 43). I share Thiele's 

 opinion, who does not regard the so-called mouth-cavity as a part of the digestive 

 tract, but considers it an independent ectodermal sense-cavity (17, pp. 308 and 314). 

 A proper mouth-cavity occurs in the proximal part of the digestive tract, which is 

 slightly dilated, and the wall of which presents numerous folds. The epithelial cells 

 of which the wall of this part is composed are very transparent and have large oval 

 nuclei. The wall is surrounded by a thin circular muscular layer. The folds, as it 

 appears to me, allow this portion of the digestive tract to extend considerably after 

 the taking of food. I do not believe them to be sensitive (Figs. 6 and 7, pli). 

 Gradually this portion passes into the following part of the alimentary canal. The 

 latter is much wider, and likewise provided with a strongly folded wall (Fig. 8, ves). 

 Its cells are taller than those of the first portion, and have elongated nuclei ; they are 

 covered with a cuticle. Consequently these two parts of the digestive tract are easily 

 distinguished from one another. More distally, however, between these slender cells 

 there occur broader ones with more oval nuclei, the cuticle no longer being easily 

 discerned throughout. Finally the spacious intestine unites with the proximal caecum 

 (Fig. 8, c). 



In the second portion of the digestive tract the radula, which is of a very 

 peculiar structure, is found. In the digestive tract a proximally directed cone appears 

 with a strong ventral fold (Fig. 8, en). This cone is beset throughout with radula- 

 teeth (Fig. 12, c). For its further structure compare figures 12-14 and 8-11. An 

 epithelial layer appears (a), which covers the radula. The ventral wall of the 

 digestive tract {b) unites with the ventral wall of the cone (c) ; the epithelial layer 

 a with the side-walls of the cone. The two lateral walls of the proximal part of the 

 alimentary canal unite dorsal from the cone. By this complicated process the cone, 

 which on the outside is beset with small teeth, is divided into two closed pouches ; the 

 latter are on the inner side closely beset with teeth and are situated outside the 

 alimentary canal. The dorsal pouch is the radula-sac, the ventral contains adult 

 radula-teeth and, distally, ends blindly. Consequently the ventral pouch is but 

 a ventral caecum of the proximal part of the digestive tract, directed distally ; 

 teeth on the ventral wall are wanting, in the distal part teeth only occur on 

 the dorsal wall. The radular teeth are well developed (Fig. 16). They are 

 triangular, with a broad base, and more or less strongly curved, often recurved 

 at the point (Fig. 17; B). Frequently forms occur as represented in Fig. 17, A; 

 often other forms are met with, which, however, I consider fragments of whole 

 teeth. Fig. 17, C shows a tooth as seen from the ventral side : the base 

 appears to be broader than the curved, part. The teeth are placed in irregular 

 rows, and rest in crypts of the epithelium (Fig. 16). A basal membrane is 

 present. This is remarkable ; Thiele lays stress on the absence of such a membrane 



B 2 



