WITH NOTES ON THE WEST INDIAN SPECIES. 333 



Fig. 50. Similar section at a more posterior level (behind the ventral coelomic canals), 

 showing the post-trematic coecal projection of the end-sac into the anterior end of the left 

 perihaerual cavity. This section with the next shows the inclusion of the fused anterior 

 portions of the cornua of the nuchal skeleton within its body, which presumably takes place 

 concomitantly with the progressive growth in length of the animal. 



Fig. 51. Portion of section shortly behind the insertion of proboscis into collar, showing 

 the medullary tube (which opens at a slightly more anterior level, independently, by the 

 anterior neuropore) and the anterior epidermal involution (anterior Epidermistasche), formed 

 by backward extension of the angle of insertion of proboscis and collar, dorsal to the 

 medullars tube with which it is united by a short septum. 



PLATE XXXII. 



Figs. 52 — 60 refer to Sp. alba. 



Fig. 52. Portion of section through region of transition from collar to trunk, but in 

 front of the posterior neuropore. On the left of the figure the truncal canal is seen 

 communicating with the first gill-pouch at the commencement of the first gill-slit and at 

 the level of the collar-funnel. On the right of the figure the section passes in front of the 

 truncal canal of that side. 



Fig. 53. Section through one of the perihaemal cavities immediately in front of a 

 truncal canal. At x is seen a distinct loculus for the reception of the truncal canal ; this 

 loculus extends over several sections. 



Fig. 54. Section through the same perihaemal cavity at a level intervening between 

 Fig. 52 and Fig. 53, showing the truncal canal as an independent tube. 



Fig. 55. Section through branchial region. On the left a tongue-bar is shown, and on 

 the right the figure shows the entire half of a gill-slit between septal bar and tongue-bar. 

 Note absence of medial gonads, presence of inner circular muscles and of a minute dorsal 

 diverticulum of the gill-pouch {dgp). 



Fig. 56. Upper half of section through the branchiogenital transition, showing accessory 

 gonads and last gill-slit. 



Fig. 57. Similar section through posterior end of genital region (genito-hepatic transition), 

 showing intestinal canals and pores (vestigial gill-slits). Owing to the overlapping of these 

 canals, as described in the text, portions of no less than five of them are seen to the left 

 of the figure and two on the right. They consist of ectodermal and endodermal portions, 

 and occur at the dorsal angles of the gut, in the same position as the terminal true gill- 

 slits (cf. preceding figure). 



Fig. 58. Section through the hepatic region, showing the gonads continued into this 

 rewion, the internal hepatic saccules (that on the right cut tangentially), an intersaccular 

 epidermal involution, the lateral longitudinal vessels and the ventro-lateral epidermal tracts. 



Fig. 59. Portion of section through epidermis of genital region, to show the depth of 

 the dermal pits in this species and individual. 



Fig. 60. Ventral portion of section through caudal region, to show the thickened median 

 \eutral wall of hind-gut representing the pygochord. 



