278 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



There is no such glandless tract over the greater part of the abdominal region 

 (60 mm. in length), but its place is taken on each side by a shallow groove which 

 is not associated with absence of glandular ridges. The effect of these grooves is to 

 raise the median ventral tract into a prominent rounded keel. The caudal region 

 (about 14 mm.) is slightly swollen and is only distinguished externally by the flattening 

 out of the above-described grooves and the consequent absence of a ventral keel. 



PROBOSCIS. 



The central cavity is sharply defined and extends nearly to the tip of the 

 proboscis. Anteriorly it is surrounded by a uniform layer of felted fibres and it is 

 difficult to distinguish the dorsal and ventral sides of the cavity. Farther back the 

 decussation of the fibres denotes the dorsal side. But from the extreme front end of 

 the proboscis the ventral middle line is defined by the presence of the ventral 

 recurrent vessel which lies immediately inside the layer of circular muscles and 

 communicates at frequent intervals with the epidermal system of blood-vessels (i.e. the 

 blood-spaces in the basement-membrane of the epidermis) by perforating the circular 

 musculature. A similar recurrent vessel is present in S. porosa, where it rises up to 

 a more dorsal position below the vermiform process. In the present species the 

 recurrent vessel retains its ventral position adjacent to the circular musculature until 

 it passes up along the free edge of the ventral septum (PL XXXI. Figs. 46 — 47). 



The dorsal recurrent dermal vessel runs, as is its wont, along the anterior edge 

 of the pericardium in front of which its distal portion forms a vascular complex. 

 (PI. XXXI. Fig. 47). 



The circular musculature of the proboscis is generally thicker than the nervous layer 

 of the epidermis. 



The vermiform process of the stomochord is of varying calibre and in its 

 anterior portion there is, in one or two places, an actual discontinuity, as if a certain 

 amount of fragmentation had taken place. It is solid and its cells are undifferentiated 

 in the anterior two-thirds, becoming vacuolar as the body of the stomochord is 

 approached. The vermiform process is supported in the thin median septum of the 

 proboscis, and the dorso-ventral muscles are inserted into the basement-membrane 

 surrounding it (PI. XXXI. Fig. 46). The vermiform process passes quite gradually into 

 the body of the stomochord and it is impossible to say where one begins and the other 

 ends. 



The pericardial auricles are very minute, almost non-existent, but the glomerulus 

 projects beyond the anteiior limit of the pericardium for a moderate distance as 

 paired glomerular horns at the sides of the stomochord. 



The ventral septum has a forwardly directed free edge and does not extend 

 to the base of the vermiform process or to what might be considered as such but 

 falls somewhat behind the anterior region of the main body of the stomochord. 



In the coecal region of the stomochord the lateral pouches are extremely well- 

 marked and tend to project slightly forwards as independent pouches. Unlike S. porosa, 

 their cavities do not communicate across the middle line but remain separate until 



