WITH NOTES ON THE WEST INDIAN SPECIES. 



259 



pouches of the stomochord. "From the transverse himen of the blind sac," says Hill, 

 "there passes forwards laterally a short horn on each side; a section passing through 

 the proboscis neck just anterior to the passing down of the ventral blind sac lumen 

 thus shows three cavities in the notochord," namely, two ventro-lateral and one dorso- 

 median. 



In Pt. hedleyi a similar condition has been described also by Hill. Here, in the 

 region of the ventral blind sac the stomochord is transversely extended and "somewhat 

 dorso-ventrally compressed. From the lumen of the blind sac there pass forwards two 

 short lateral horns" as in Pt. australiensis. 



In the mid-coecal region of the stomochord the latter has a characteristic helmet- 

 shape in section, the ventro-lateral corners arching downwards over the ventral coelomic 

 canals. In the median dorsal division of the helmet-shaped stomochord are numerous 

 mucous gland-cells (PI. XXX. Fig. 26). 



Behind the coecal region, the character of the stomochord changes entirely. It 

 is greatly reduced in bulk and its walls become thin and are apparently in a 

 condition of mucous degeneration. In fact, the function of the stomochord as a 

 supporting structure, a function which it undoubtedly serves in its anterior moiety, 

 is, in the nuchal region, quite superseded by the nuchal skeleton. 



The nuchal region of the stomochord is therefore in a retrograde condition, and 

 we are prepared for the fragmentation described above in Pt. carnosa and by Spengel 

 in Bal. kupfferi ; and for the entire resorption of the nuchal portion of the stomo- 

 chord which Spengel has described in old examples of Sch. hrasiliense and in Bal. 

 canadensis. 



Proboscis-pore. 



The dorsal proboscis-canals occur above the middle or coecal region of the 

 stomochord, being separated from the ventral canals by the lateral pockets of the 

 dilated stomochord and from each other by the dorsal wall of the pericardium, which 

 meets the basement-membrane of the epidermis, as in other species (PI. XXX. Fig. 2.5). 



There is only one proboscis-pore, namely, that on the left side (PI. XXX. Fig. 27). 

 The left dorsal canal leads into an end-vesicle which lies in a median position above 

 the pericardium, the apex of which is therefore shifted over to the right side. 



The pore is coterminous with the end-vesicle, or, in other words, there is no 

 coecal extension of the end-vesicle behind the pore. In different specimens there is 

 great variation in the topographical relations of the various organs in the neck of 

 the proboscis. Thus, in one case the left dorsal coelomic canal opens into the end- 

 vesicle at the commencement of the coecal dilatation of the stomochord, i.e. at the 

 commencement of the lateral pouches of the stomochord. The medianly placed end- 

 vesicle accompanies the pouched region of the stomochord throughout its length, and 

 opens by a sinistral pore at the transition from the coecal to the nuchal portion of 

 the stomochord or, what is the same thing, at the transition from the cupule to the 

 body of the nuchal skeleton ; the pore therefore occurs well in front of the alary 

 processes of the skeleton and in front of the posterior edge of the ventral septum, 

 which, in this case, extends back into the free lobe described below. 



