332 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



Pig. 39. Section passing through the proboscis-pore (external opening of end-sac). It 

 may be noted how the substance of the nuchal skeleton grades off into the chondroid tissue. 

 Above the stomochord the dorsal vessel is seen to give off the advehent dermal vessel of 

 the proboscis, on either side of which are the anterior extremities of the perihaemal cavities. 



Fig. 40 a — c. Sections through the vestigial root of Sp. porosa. 



Fig. 40 a shows the distal terminal vesicle with its thick wall enclosing mucoid contents. 

 Zeiss Oc. 3, Obj. D. 



Fig. 40 b (posterior to preceding and less highly magnified) shows the root itself abutting 

 upon and terminating in the distal vesicle, the wall of which is cut tangentially, and encloses 

 a homogeneous mass of darkly staining matter. 



Fig. 40 c (still farther back) shows the basal portion of the root and its origin from 

 the dorsal wall of the medullary cord. 



Fig. 41. Section through the middle of the collar nerve-cord (medullary cord), showing 

 medullary cavities and distribution of fibrous layer. 



Fig. 42. Section through collar-canal. 



Fig. 43. Cross-section of the skeletal rod of a septal bar with blood-vessel at inner 

 end ; taken from a horizontal section through the pharynx. 



Fig. 44. Section through posterior end of collar with posterior neuropore, showing the 

 truncal canals in the perihaemal cavities. On the left the figure shows the truncal canal 

 opening into the first gill-pouch at the level of the collar-funnel ; on the right the truncal 

 canal is seen at a more anterior level, and the posterior end of the peripharyngeal cavity 

 of that side is seen lying against the wall of the throat (collar-gut), while imbedded within 

 the wall is the posterior extremity of one of the cornua of the nuchal skeleton. 



Fig. 45. Section through the branchial region, showing medial gonads, inner circular 

 muscles and oesophageal portion of gut. On the right is shown a tongue-bar and on the 

 left a septal bar with the cut ends of the synapticula. 



Figs. 46 — 51 refer to Sp. alba, n. sp. 

 Fig. 46. Portion of section through proboscis in front of central complex (cf. Fig. 36). 



Fig. 47. Portion of section through commencement of central complex. The section 

 passes immediately in front of the anterior end of the pericardium, and shows the two 

 anterior horns of the glomerulus on either side of the stomochord. Above the latter is 

 seen the vascular complex which constitutes the dorsal recurrent dermal vessel of the proboscis. 



Fig. 48. Section through base of proboscis passing through the pouched region of 

 stomochord, shortly behind the glomerulus, the continuation of which into the efferent vessels 

 is seen below the ventral epithelium of the dorsal coelomic canals (cf. PI. XXX. Fig. 25). 

 The pericardium is seen to be traversed by transverse fibres, and above it are the two 

 primary branches of the advehent dermal vessel. The dorsal and ventral coelomic canals 

 are separated by the lateral pouches of the stomochord. 



Fig. 49. Section through the neck of the proboscis in the region of the proboscis-pore, 

 passing in front of the posterior termination of the ventral coelomic canals. The chondroid 

 substance is homogeneous, and is characterised by the relative scarcity of cellular islets. 



