232 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



Stomochord, Pericardium and Glomerulus. 



The behaviour of the stomochord at its distal free extremity, and its relations with 

 the pericardium and glomerulus at this point, are of importance both morphologically 

 and systematically. In Pt. flava the stomochord is attenuated at its distal end, being 

 drawn out into a narrow, solid, cellular cord, with which the pericardium and glomerulus 

 are exactly coextensive. The pericardium, like the stomochord, has a simple, pointed 

 anterior extremity. 



These three structures (stomochord, pericardium and glomerulus) constitute together 

 the central complex of the proboscis. Corresponding to the ventral septum of the 

 proboscis there is, as already mentioned, on the dorsal side a hollow septum, formed 

 by the pericardium, reaching up to the dorsal integument. Its anterior point of union 

 with the basement-membrane of the epidermis, or, in other words, the point (regarding 

 from behind forwards) at which the pericardium ceases to reach the skin, and com- 

 mences to stretch with a gently inclined free dorsal border to the anterior tip of the 

 stomochord, is far removed from the distal end of the central complex, occurring 

 slightly in front of the level of the anterior free edge of the ventral septum. 



The cavity of the pericardium, in the preparations examined by me, is rarely 

 completely filled up by proliferation of its endothelial lining, although the extent to 

 which such proliferation has occurred varies in different specimens. Perhaps it varies 

 at different ages or different periods. In one specimen the anterior end of the peri- 

 cardium was practically filled with a spongy, reticulate tissue. 



The median septum of the proboscis is principally formed by what Spengel has 

 described as the doi'so-ventral muscle-plate. Its relations to the central complex are 

 of some systematic importance. In Pt. flava the median septum is essentially co- 

 extensive with the central complex, and does not extend in front of the latter. 



The stomochord of the Enteropneusta may be resolved into three distinct regions, 

 each of which may present features of diagnostic value. These are (1) the anterior 

 or interglomerular region, (2) the middle or coecal region, and (3) the posterior or 

 nuchal region. 



In the kSpengelidae the anterior portion is produced into a long, vermiform process, 

 but in other Enteropneusta it is, generally speaking, coextensive with the glomerulus. 



If we examine the stomochord of Pt. flava from before backwards we find that 

 the anterior attenuated cellular cord passes gradually into a wider portion, with stellate 

 lumen ; the stomochord then gradually attains a certain thickness in the dorso-ventral 

 direction, so that, in section, it appears oval or elliptical in shape ; the lumen mean- 

 while becomes indefinitely subdivided. Farther back, near the commencement of the 

 ventral septum, the lumen becomes single and well-defined, and the transverse diameter 

 of the stomochord nearly equals its dorso-ventral diameter. 



The coecal dilatation (i.e. the ventral " Blindsack " of Spengel) which characterises 

 the middle region of the stomochord has no continuous transverse lumen in adult 

 examples of Pt. flava, but there may be a trace of such a lumen in a more or 

 less occluded condition. The lateral portions^ of the lumen appear as paired diverticula 

 from the principal lumen. 



' In some species, rather more so than in Pt. flava (e.g. Pt. ruficollis and in Spengelia), the lateral portions 



