272 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



which stains black with haematoxylin. The end-vesicle of this vestigial root would in 

 fact seem to be in a condition of mucoid degeneration. The root itself contains a 

 well-marked, though interrupted vestige of an axial canal, which however does not com- 

 municate with any of the medullary cavities of the cord, although immediately behind 

 and at the base of the root there occurs a comparatively large medullary cavity. 



The posterior portion of the collar nerve-cord, like the anterior, contains a broad 

 continuous lumen and it is at the commencement of this posterior lumen that a low 

 hollow pouch-like diverticulum arises to the 'right of the middle line. It is quite 

 short and does not appear in section separate from the nerve-cord. It is open to 

 anyone to regard this structure as a second vestigial root, to which, however, no special 

 interest attaches. 



Peripharyngeal Cavities. 



In my preliminary diagnosis of this species I stated that there were no peri- 

 pharyngeal cavities. Having since been so fortunate as to obtain a second species 

 of the genus, my examination of it led me to re-investigate S. porosa in respect of 

 this quality and I find that there is a pair of peripharyngeal cavities in a vestigial 

 condition. 



Their vestigial nature is clearly established by the fact that they are closed 

 behind as well as in front and therefore do not communicate with the body-cavity 

 of the trunk. 



In Schizocardiuvi, Spengel has shown that the right and left peripharyngeal spaces 

 are triangular in shape, the apex of the triangle being directed forwards and reaching 

 to the level of the point of bifurcation of the nuchal skeleton; the base of the triangle 

 lies at the posterior end of the collar where the cavity enters into free communication 

 with the truncal coelom. 



This condition is indeed met with in Sjiengelia alba n. sp. In the present species, 

 however, the peripharyngeal cavities are pointed at both ends, sharply pointed in front 

 and bluntly behind. The anterior extremity reaches to a point shortly behind the 

 level of the bifurcation of the nuchal skeleton. As the cornua of the latter divaricate, 

 the dorso-ventral extension of the peripharyngeal cavity (on each side) increases, until 

 a maximum is reached in the mid-region of the collar. Then reduction commences ; 

 the dorsal edge of the cavity, which is always separated by an interval from the edge 

 of the perihaemal cavity of its side, recedes further and further from the vicinity of the 

 latter, the ventral extension of the cavity becomes likewise reduced until the entire 

 cavity ceases some distance in front of the termination of the nuchal skeleton. The 

 posterior end of the cavity lies against the throat-epithelium half-way between the 

 dorsal and ventral sides of the latter (PI. XXXI. Fig. 44). 



Collar-coelom and Pores. 



There is no dorsal septum in the collar, except the fold of basement-membrane 

 associated with the first vestigial root, which is probably to be regarded as a vestige 

 of the dorsal septum. On the other hand, the ventral septum has an unusual forward 



