WITH NOTES ON THE WEST INDIAN SPECIES. 



281 



The epibranchial band is markedly cristate, the epithelium being thickened in 

 the median line ; on the inner surface of the band there is a shallow median 

 longitudinal groove opposite to the crest. The groove flattens out and the crest 

 becomes broader at intervals corresponding with the breadth of a gill-cleft. 



The gill-bars are only slightly arcuate. At the medial dorsal angle of every gill- 

 pouch there is a very small diverticulum (PL XXXII. Fig. 55). This minute diverti- 

 culum of the gill-pouch occupies a position corresponding to that of the large 

 truncal canals described above. It is of course not peculiar to this species, but is 

 particularly well-defined here. It is in such a position that the nephric tubules 

 occur in Amphioxus ; and it is possible that at the dorsal angles of the gill-pouches 

 of Enteropneusta we have the makings or the primordia of nephric tubules. 



The first gonad on the right side is quite unripe, and I am unable to say 

 whether it is in an incipient or in an arrested state of development ; it is connected 

 with the ectoderm between the gill-clefts V and VI. 



The second gonad is fully formed and contains ripe spermatozoa ; its duct occurs 

 between VII and VIII ; the third genital duct is between VIII and IX, and so 

 forth. On the left side there is no unripe anterior gonad like that on the right ; 

 the first duct is between VI and VII, the second at the level of VIII, the third 

 between IX and X, and so on. 



I have estimated that there are approximately 80 gill-pores on each side. Each 

 half of each gill-cleft is traversed by 10 — 11 synapticula. 



The branchial groove commences in front as a narrow sulcus which gradually 

 widens out posteriorly so that the gill-pores are plainly visible with a simple lens 

 (PI. XXVII. Fig. 9 a). At its hinder end the groove is as much as "75 mm. in 

 breadth ; the gill-pores lie close against the submedian ridge leaving a smooth epidermal 

 tract to form the floor of the branchial groove between the line of pores and the 

 upper margin of the lateral annulations. 



Branchiogenital Transition and Genital Region. 



Behind the branchial region the branchial groove is continued into the genital 

 region, not however as a continuous groove but as an interrupted groove traversed by 

 dermal bridges. In this way there is produced the appearance of a series of dermal 

 pits which, as already noted, present externally the same aspect as the dermal pores 

 of S. porosa with which they are evidently homologous, though they are much shallower 

 than the latter (PI. XXXII. Fig. 59). There are upwards of 25 of these dermal pits 

 on each side (PI. XXVII. Fig. 9). 



In the branchial region there are no medial gonads, i.e. no gonads raediad of 

 the gill-pores, and in the anterior moiety of this region the gonads form a simple 

 lateral series on each side. In the posterior part of the branchial region where the 

 branchial groove widens out, as described above, accessory gonads begin to appear 

 in the space which lies between the main series of lateral gonads and the gill-pores. 

 There may be as many as three accessory gonads in one plane, but they are irregular 



39—2 



