WITH NOTES ON THE WEST INDIAN SPECIES. 



279 



they die out (PI. XXXI. Fig. 48). Other minute cavities or vestiges occur in the body 

 of the dilated stomochord in addition to the main lumen, which is also interrupted. 



The ventral canals terminate in the chondroid tissue without communicating 

 with each other, as in Glandiceps. 



Of the dorsal canals only the left communicates with an end-vesicle which is 

 sinistral in position (not quite median) and opens by a short narrow sinistral pore to 

 the exterior (PI. XXXI. Fig. 49). Behind the proboscis-pore the basement-membrane 

 surrounding the vesicle closes in once more and the end-vesicle is continued for a 

 relatively long distance (about 100 fx) as a coecal tube, the posterior end of which 

 actually projects into the anterior end of the left perihaemal cavity^ (Pi. XXXI. Fig. 50). 



The prae-trematic, post-trematic and trematic behaviour of the end-vesicle of the 

 proboscis canals is of the very greatest importance to anyone willing to penetrate 

 into the morphological tangle surrounding these structures. The post-trematic pro- 

 longation of the end-vesicle of S. alba is therefore worthy of particular note as being 

 one of the most striking characters of the species. 



The nuchal skeleton resembles that of S. porosa in the main. Its principal 

 characters are sufficiently shown in the figures to obviate a detailed verbal description 

 (PI. XXXI. Figs. 49—51). 



COLLAR. 



Not only is the collar musculature (inner longitudinal muscles and perihaemal 

 muscles) projected into the neck of the proboscis but the anterior neuropore also 

 occurs at the posterior end of the nuchal region and is independent of the duplica- 

 ture of the collar (PI. XXXI. Fig. 51). In most other species the two structures 

 coincide (see below p. 304). The central canal leading backwards from the neuropore 

 only extends for a short distance, after which the medullary cord is nearly solid, with 

 numerous disconnected vestiges of medullary cavities mostly ill-defined. There may be 

 distinguished two main lateral series of cavities with irregular intervening vestiges. 

 The posterior central canal leading to the posterior neuropore is much longer and 

 more capacious than the anterior canal. There are no roots of any kind. 



The dorsal septum extends to the anterior end of the collar nerve-cord appearing 

 immediately behind the anterior neuropore and joining the cord with the basement- 

 membrane of the epidermal iovolution which is associated with the collar-duplicature 

 (PI. XXXI. Fig. 51). Posterior to this involution the septum is present but does not 

 reach the epidermis until the level of the buccal orifice of the stomochord is reached, 

 after which its course is uninterrupted to the posterior end of the collar. 



The collar canals have the usual semilunar funnel behind which the dorsal 

 wall is invaginated into the lumen of the canal. The dorsal plication is characterised 

 by its tenuity due to the low cells composing it ; the remaining walls of the canals 

 consist of high columnar epithelium. The canals opeu into the first gill-pouch in 

 the normal manner. 



1 The perihaemal cavities project forwards for a short distance into the neck of the proboscis. 



w. III. 39 



