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THE ENTEROPNEUSTA. 641External features. An exceedingly small form. The proboscis is nearly r5mm. inlength and is somewhat longer than the collar (PI. XXXVIII. fig. 18). The branchial regionis 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 lessthan 1 mm. wide. The anterior border of the collar is rather \vider than the rest of thispart of the animal and is somewhat crinkled. Posteriorly the collar shews a well-markedcircular furrow (PL XXXVIII. fig. 18, t.f.). The genital pleurae are small and are widelyseparated from each other where they join the collar. The mature and swollen gonads in thisspecimen cause the edges of the pleurae to assume a somewhat beaded appearance.Proboscis. The proboscidial coelom is almost entirely filled by connective tissue andmuscle fibres. A delicate layer of longitudinal muscle fibres is found beneath the basementmembrane, but circular fibres are absent. Traces of a dorsal septum are to be found slightlyin front of the pericardial sac. The ventral septum is complete at a level shortly beforethat at which the glomerulus ends ; it carries the ventral glomerulus vessels. The ventralproboscis coelom is packed with, muscle fibres and connective tissue except at its extremeposterior end where the ventral septum is lacking (PI. XXXVIII. fig. 22, v.c). A racemoseorgan is not present.A single median proboscis pore is present, placing the left division of the dorsal probosciscoelom in communication with the exterior (PI. XXXVIII. fig. 22, p.p.). The right moiety ofthe dorsal proboscis coelom ends blindly. Throughout its whole extent the stomochord shewsa well-marked lumen. The lateral diverticula of the stomochord are small (PI. XXXVIII.fig. 22, S.C.). The nuchal skeleton is provided with a deep narrow keel (PI. XLI. fig. 3, sk. 1).The chondroid tissue in this region is exceedingly scanty.Collar. The collar epithelium is very thick (PI. XXXIX. fig. 24, ep.). The glandular zonecommences at the level where the cornua of the nuchal skeleton terminate and reaches tothe commencement of the collar funnels. The outer longitudinal muscle layer is well-marked,particularly in the anterior and middle regions of the collar (PI. XXXIX. fig. 24, el.). Theinner 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 fora very small space on either side of the dorsal septum. This last-named structure is firstfound immediately behind the first dorsal root in the middle of the collar region. Tracesof a ventral septum exist but in no place is it complete. The collar funnels are of theusual form and open into the first gill pouch.The cornua of the nuchal skeleton extend half-way round the oesophagus. In lengththey are short, being only about i of the total collar length.The collar cord is tubular containing a single axial canal (PI. XXXVIII. fig. 15) surroundingwhich are the ganglion cells. The layer of dorsal ganglion cells (PI. XXXVIII.fig. 15, gcd.) is thin, being best marked in the region where the first and second dor.salroots are given off. Above it is a much attenuated layer of nerve fibrils. The ventralganglion cells {gcv.) are much more numerous and in transverse section are seen as a V-shapedmass shewing a bifurcation at the point of the V. The layer of nerve fibrils surroundingthem is well-developed. Separating the dorsal and ventral ganglion cells on either side isa layer of large oval vacuolated cells—probably gland cells from which the secretion hasbeen ejected or dissolved out. Some deeply staining mucoid substance is to be found inthe medullary canal. Here and there occur ganglion cells rather larger than the rest,