THE ENTEROPNEUSTA. 641 
EXTERNAL FEATURES. An exceedingly small form. The proboscis is nearly 15mm. in 
length and is somewhat longer than the collar (Pl. XX XVIII. fig. 18). The branchial region 
is about half as long again as the collar. Width of collar and proboscis about 1°5 mm. 
The width of the branchial region is not quite so great, whilst the genital region is less 
than 1 mm. wide. The anterior border of the collar is rather wider than the rest of this 
part of the animal and is somewhat crinkled. Posteriorly the collar shews a well-marked 
circular furrow (Pl. XXXVIII. fig. 18, tf). The genital pleurae are small and are widely 
separated from each other where they join the collar. The mature and swollen gonads in this 
specimen cause the edges of the pleurae to assume a somewhat beaded appearance. 
Proboscis. The proboscidial coelom is almost entirely filled by connective tissue and 
muscle fibres. A delicate layer of longitudinal muscle fibres is found beneath the basement 
membrane, but circular fibres are absent. Traces of a dorsal septum are to be found slightly 
in front of the pericardial sac. The ventral septum is complete at a level shortly before 
that at which the glomerulus ends; it carries the ventral glomerulus vessels. The ventral 
proboscis coelom is packed with muscle fibres and connective tissue except at its extreme 
posterior end where the ventral septum is lacking (Pl. XXXVIII. fig. 22, vc.). A racemose 
organ is not present. 
A single median proboscis pore is present, placing the left division of the dorsal proboscis 
coelom in communication with the exterior (Pl. XXXVIII. fig. 22, p.p.). The right moiety of 
the dorsal proboscis coelom ends blindly. Throughout its whole extent the stomochord shews 
a well-marked lumen. ‘The lateral diverticula of the stomochord are small (Pl. XXXVIII. 
fig. 22, s.c.). The nuchal skeleton is provided with a deep narrow keel (Pl. XLI. fig. 3, sk. 1). 
The chondroid tissue in this region is exceedingly scanty. 
Collar. The collar epithelium is very thick (Pl. XX XIX. fig. 24, ep.). The glandular zone 
commences at the level where the cornua of the nuchal skeleton terminate and reaches to 
the commencement of the collar funnels, The outer longitudinal muscle layer is well-marked, 
particularly in the anterior and middle regions of the collar (Pl. XXXIX. fig. 24, el.). The 
inner longitudinal layer is also strongly developed. No circular muscle fibres are present. 
The collar coelom is entirely filled with muscle fibres and connective tissue except for 
a very small space on either side of the dorsal septum. This last-named structure is first 
found immediately behind the first dorsal root in the middle of the collar region. Traces 
of a ventral septum exist but in no place is it complete. The collar funnels are of the 
usual form and open into the first gill pouch. 
The cornua of the nuchal skeleton extend half-way round the cesophagus. In length 
they are short, being only about } of the total collar length. 
The collar cord is tubular containing a single axial canal (Pl. XX XVIII. fig. 15) sur- 
rounding which are the ganglion cells. The layer of dorsal ganglion cells (Pl. XXXVIII. 
fiz. 15, ged.) is thin, being best marked in the region where the first and second dorsal 
roots are given off. Above it is a much attenuated layer of nerve fibrils. The ventral 
ganglion cells (gcv.) are much more numerous and in transverse section are seen as a V-shaped 
mass shewing a bifurcation at the point of the V. The layer of nerve fibrils surrounding 
them is well-developed. Separating the dorsal and ventral ganglion cells on either side is 
a layer of large oval vacuolated cells—probably gland cells from which the secretion has 
been ejected or dissolved out. Some deeply staining mucoid substance is to be found in 
the medullary canal. Here and there occur ganglion cells rather larger than the rest, 
