Contributions to the Anatomy and Histology of Thalassema neptuni Gaertner. 545 



The nucleus, which hes iu a slightly projecting mass of proto- 

 plasm over which the sarcolemraa also extends, is highly granular 

 and vacuolated in structure. The alveolar ladder -like structure of 

 the cortical layer of these fibres was iu some macerated preparations 

 so very plain that I have figured it (Fig. 9) ; the preparation was 

 stained with haematoxylin and eosin. On transverse sections occasionally 

 a similar structure was visible. 



The peritoneal lining of the body cavity is very firmly 

 adherent to the musculature, where its attachment appears to be to the 

 intermuscular connective tissue. It is, as is well known, composed of 

 a layer of lenticular cells of somewhat polygonal outline in surface 

 section. The nuclei are ovate, and do not stain deeply ; the protoplasm 

 also as a rule stains lightly in comparison to the adjoining tissues. 



III. Digestive System. 



The mouth lies at the base of the proboscis, the lateral margins 

 of which meet but do not fuse ventrally. That is to say the under 

 lip belongs to the body and not to the proboscis. From the mouth 

 to the anus, which is, as in all Echiuroids , terminal, the alimentary 

 canal forms a number of complicated twinings (Fig. 11), being slung 

 to the body wall throughout its entire length by a dorsal mesentery, 

 and for a short distance anteriorly and posteriorly by a ventral 

 mesentery. These mesenteries will be considered later. The digestive 

 tube can best be divided into 1) an anterior division or fore gut, the "in- 

 testin buccal" of RiETSCH (12) corresponding to the Pharynx, Oeso- 

 phagus and Crop of Spengel (17), in which the longitudinal muscu- 

 lature, lies outside the circular musculature, 2) an intestine proper, with 

 reversed musculature, characterized by the presence of the ciliated 

 groove, and in part of its course by the collateral intestine, and 3) 

 a short rectum. On the anterior division or fore-gut I recognise four 

 distinct regions, a pharynx, oesophagus, gizzard and crop. 



The oesophagus and gizzard together correspond to the oeso- 

 phagus of Echiurus , a functional differentiation into glandular and 

 muscular parts having in Thalassema neptuni led to a marked structural 

 difference (Fig. 12 oe and g). I retain the word crop, applied by 

 Spengel in Echiurus, for the very characteristic division immediately 

 before the intestine (fig. 12 or), in the absence of a better name. As 

 I will demonstrate later on, this division presents histological characters 

 which warrant us in attributing to it an active secretive function. 

 The crop (cr) is separated from the intestine by a marked constriction. 



