WITH NOTES ON THE WEST INDIAN SPECIES. 



237 



tudinal ridge, which Spengel has called the " oesophageale Grenzwulst." These ridges 

 are what I call the parabranchial ridges, and I am disposed to attach high morpho- 

 logical importance to them. They are certainly important structures anatomically, as 

 may be best seen by simple dissection (PI. XXVIII. Fig. la). They are co-extensive 

 with the gill-slits, and, at the anterior end of the pharynx, they are seen to pass 

 round into the median dorsal epibranchial band. As shown in PI. XXVIII. Fig. 8, the 

 parabranchial ridges are nothing else than the confluent thickened ventral ivalls of tlie gill- 

 clefts. 



Genital Pleurae and Lateral Septa. 



The genital pleurae of Pt. flava resemble those of the two species of the sub- 

 genus Chlamydothorax which were described by Spengel, in their extreme ventral 

 origin. So low is their origin that they leave the pharynx quite free and exposed, 

 so that the gill-slits open laterally in their entirety directly to the exterior. The 

 pharynx remains erect owing to the rigidity of its walls, which is effected by the 

 skeletal supports in the gill-bars. The latter are markedly arcuate with the convex 

 side directed outwards so that the pharynx presents the appearance of a sub- cylin- 

 drical body with the annular septal branchial arches and intervening slits. The tongue- 

 bars lie far inwards towards the cavity of the pharynx, and so are not visible in 

 external view in preserved specimens. In all Ptychoderidae the tongue-bars have a 

 more internal position than the septal bars. 



The gill-clefts are crossed by solid connecting rods or synapticula, so that the 

 result is a branchial basket (PI. XXVIII. Fig. 8). 



The average number of synapticula on each side of a tongue-bar is 10 — 12; 

 but there may be as many as 18 — 20 in the macrobranchiate forms. The plication 

 of the outer wall of the tongue-bar, which has been noted in so many cases, is not 

 a constant feature in Pt. flava. It not only varies in successive bars but at different 

 levels in the same bar. 



In most Enteropneusta the external openings of the gill-slits are reduced to small 

 pores, which occur in linear sei'ies on the dorsal side of the animal, on each side of 

 the middle line, at the base of a longitudinal groove known as the branchial groove ; 

 in such cases the gill-slits no longer open directly to the exterior but into special 

 branchial pouches which, in their turn, discharge to the exterior by the aforesaid 

 gill-pores. 



The genital pleurae, in addition to their primary function of bearing the gonads, 

 serve also the accessory function of j^rotecting the branchial basket. They can be 

 folded over the latter so as to meet and even overlap in the dorsal middle line, and, 

 when so carried, they form a complete temporary peribranchial chamber, only open 

 posteriorly where the pleural folds divaricate. On the other hand they can be spread 

 out laterally in the plane of the ventral side of the body. They are extremely mobile 

 structures, and attain their maximum development about the middle of the branchio- 

 genital region. 



Towards the hepatic region the pleurae diminish gradually in size and also attain 

 a more dorsal position. They enclose the more anterior liver-saccules, and are finally 



