252 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



described above. The alary processes are produced in front of the keel and bound the 

 dorso-lateral sides of the ventral coecum of the proboscis-cavity, behind which they unite 

 to form the keel. In the anterior region of the keel, the body of the skeleton is reduced 

 to a thin flattened vertical band which becomes, for a short distance, absorbed into 

 the substance of the keel, being almost entirely replaced by the latter. Farther back, 

 the body again seems to assert itself, and the keel becomes reduced until the point 

 of bifurcation is reached. 



COLLAR. 



Collar Nerve-cord and Roots. 



At its anterior end the collar nerve-cord possesses a simple transverse lumen 

 proceeding from the anterior neuropore. This soon ceases and the central canal is 

 thereafter represented by a lai'ge number of separate minute medullary cavities, until 

 the neighbourhood of the posterior neuropore is approached, when a large median lumen 

 again occurs. 



The first root is hollow throughout the greater part of its free course, but it is 

 solid at its origin from the dorsal wall of the nerve-cord. It arises in front of the 

 buccal orifice of the stomochord and has a long, oblique, forwardly directed course. 

 The layer of peripheral nerve-fibres (Punktsubstanz) accompanying the root is relatively 

 thick. 



After fusing with the epidermis the lumen of the root, surrounded by an epi- 

 thelial layer of cells, is continued for some distance forwards as an intra-epidermal 

 canal lying within the thickness of the epidermis above the nervous layer. I have 

 not observed this feature in any other species. 



Immediately below the origin of the first root there is an indication of a small 

 cavity in the nerve-cord, which however is quite filled up by a drop of deeply staining 

 mucus like that which will be described below as occurring at the distal end of the 

 vestigial root of Spengelia po7'osa. 



The second root accompanies the anterior border of the dorsal septum of the 

 collar. It is likewise hollow throughout its course, but solid at its origin and its 

 central canal also runs for some distance (30 — 40 /j.) within the epidermis above the 

 nervous layer. I do not think there is an actual opening to the exterior between 

 the epidermal cells, but the latter are grouped round the distal end of this canalicular 

 prolongation of the root in such a manner as to strongly suggest that at one time the 

 root opened at the surface by a pore (PI. XXIX. Fig. 19 a and 19 6). 



The origin of the second root lies exactly at the level of the buccal orifice of 

 the stomochord. Its course, like that of the first, is directed obliquely forwards. 



In another specimen the first root is short and band-like, and its central canal 

 vestigial, but the intra-epidermal canal into which it is produced in front possesses 

 a continuous lumen and is remarkable for its great length- — about 180 fi. 



The second root is short, mostly solid and without intra-epidermal canal. A ves- 

 tigial third root is present which is reduced at origin and insertion to little more 



