250 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



PROBOSCIS. 



In the anterior portion of the proboscis there is a small central cavity surrounded 

 by a feeble aponeurosis. The cavity is not empty but contains a cellular conglomerate. 

 Similar cell-debris have been observed by Spengel in the central cavity of the proboscis 

 of Pt. minuta and elsewhere. 



Farther back, but still in front of the central complex, the central cavity disappears ^ 

 its place being taken by the decussating and radiating bundles of connective tissue fibres. 

 Immediately in front of the central complex the cavity again opens out, only to become 

 nearly filled up once more by the extraordinarily abundant splanchnotheca in which the 

 glomerulus (and central complex generally) is imbedded. 



The anterior end of the glomerulus projects beyond the stomochord and peri- 

 cardium but, in the main, the three principal components of the central complex are 

 coextensive. The stomochord ends bluntly in front and does not taper as it does in 

 Pt. flava. The pericardium ends simply, with no sign of bifurcation in front. 



There is no well-defined lumen in the most anterior portion of the stomochord, 

 but I will not undertake to deny its existence. The reason why I cannot speak 

 certainly on this point in this case- is due to the fact that the cells of the stomo- 

 chord are here inflated, more or less bladder-like, and decidedly more like chorda- 

 tissue than I have seen in some other species. 



The cavity of the pericardium is almost entirely filled by flocculent tissue 

 resembling the splanchnotheca and containing minute granules, like the latter. 



Dorso-ventral muscle-fibres accompany the central complex as usual. 



The central blood-space attains enormous dimensions, far outstripping the sub- 

 jacent stomochord. It is surrounded on all sides, except below, by a strong muscularis, 

 derived, as usual, from the endothelium of the ventral wall of the pericardium, as is 

 shown by the fact that no basement-membrane intervenes between the muscularis and 

 the pericardial tissue. 



The ventral septum is of less extent, both longitudinally and vertically, than 

 in Pt. flava. It has a posterior free border, behind which the ventral canals fuse 

 together to form a median tube, which dilates somewhat before terminating in the 

 keel of the nuchal skeleton. 



On nearing the nuchal region the musculature of the proboscis becomes more 

 and more reduced in bulk, persisting for the longest distance in the ventral walls of 

 the ventral canals. There is no circular thickening of the circular musculature at 

 the base of the proboscis such as Spengel has described for Pt. clavigera, agreeing 

 therefore in this respect with Pt. aurantiaca. 



Accompanying the termination of the muscular fibres, the lateral walls of the 

 dorsal canals and the dorsal walls of the ventral canals acquire a ciliated columnar 

 epithelium. The right dorsal canal ends blindly in the feebly developed chondroid 



1 Tins reduction of the coelomic cavity of the proboscis should be remembered in connection with the 

 behaviour of the proboscis-pore described below. 



2 There is usually no doubt one way or the other. My material of the present species is quite faultlessly 

 preserved. 



