290 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



densely packed with mucous cells and at the outer periphery (i.e. the base) of the cells 

 there is a layer of fibrous matter or " Punktsubstanz " like that which underlies the 

 epidermis (PI. XXXII. Fig. 64). This is a peculiar feature of some importance; it 

 has the same kind of importance as the observation of cilia in this portion of the 

 stomochord of Pt. carnosa, namely, a vestigial significance. 



In contrast to the massive or sub-solid condition of the stomochord in the coecal 

 region, the anterior portion of the stomochord of this species is distinguished by 

 the spacious cavity which it contains (PI. XXXII. Fig. 62). The walls surrounding 

 the large cavity are correspondingly thinner than usual, especially the dorsal wall. 

 The central cavity buds off a large number of minute cavities which lie in the thick- 

 ness of the walls. As we approach the coecal region we find the dorsal wall of the 

 stomochord drawn up into the form of a crest protruding into the central blood- 

 space ; the internal basal angles of the crest fuse together so as to constrict off a 

 smaller dorsal portion of the lumen from the wider ventral portion. Farther back 

 the ventral cavity decreases in volume, the walls becoming proportionately thicker 

 until the massive mid-coecal region is reached. When the ventral cavity of the 

 stomochord comes to an end in the anterior part of the coecal region, the stomochord 

 consists of a nearly solid mass of vacuolar reticulate tissue with, however, a small 

 dorsal lumen and minute scattered cavities (PL XXXII. Fig. 63). 



In its anterior nuchal region the stomochord appears as a flattened transverse 

 band with linear lumen, lying above the body of the skeleton ; its dorsal wall is still 

 crowded with mucous cells and the punctate matter is continued. 



In the mid-nuchal region, the stomochord is greatly reduced ; the mucous cells 

 and fibrous matter disappear and, near the commencement of the wide posterior or 

 buccal portion of the stomochord, the latter is invaded by skeletal substance to such 

 an extent as to cause an interruption of continuity, at least so far as the lumen is 

 concerned. In the posterior nuchal region the walls of the stomochord consist entirely 

 of mucous epithelium, the dorsal wall being much thicker than the ventral. It duly 

 opens into the buccal cavity at the point of bifurcation of the nuchal skeleton. 



Nuchal Skeleton. 



The cupule of the skeleton sends out irregular digitiform processes which penetrate 

 into the substance of the stomochord. One of these processes is shown in Fig. 65. 

 PI. XXXII. From the dorsal side of the massive body of the skeleton, digitiform 

 processes also project into the anterior nuchal portion of the stomochord ; by the 

 cross-fusion of such processes, portions of the stomochord become enclosed within the 

 skeleton. In addition to these stomochordal inclusions, which are confined to the 

 anterior end of the skeleton, there are extensive cellular inclusions within the body 

 of the latter. 



There are massive alary processes and a massive keel. The ventral septum of 

 the proboscis ceases close behind the level of the proboscis pore, and the ventral 

 coecum, produced by the confluence of the ventral canals, terminates at the front 

 border of the keel. 



