Gutiy Marine Lahorotory, St. Andrews. 43 



on each side of the vestibule, and thus the enlarged ends of 

 the dumb-bell are more distinctly differentiated from the 

 curved median region with its widening vestibule (Fl. IV. 

 tig. 23). At this level there are four intermediate branchial 

 slits, and the inner on each side is the more elongated, whilst 

 the conical ventral edge of the lateral enlargement is 

 stiffened by a cap of the modified hypodermic tissue. The 

 ventral collar (jacket) has now much diminished in size, but 

 the dorsal edge of tlie organ still shows a coating of the 

 modified hypoderm. Further forward the collar forms but a 

 small U, the thick layer of its hypoderm being, as formerly, 

 dorsal : the median lamella containing the vestibule is longer, 

 whilst the dilated ends are somewhat crcscentic and sliow 

 six intermediate slits. The ventral edge still has the thickest 

 cap of modified hypoderm. The vestibule has now expanded 

 laterally into a wide space at the base of the branchiae, and 

 there are seven intermediate slits, the largest being dorsal and 

 the smallest ventral in position. Advancing forward, or 

 distiilly, the slits increase to nine, and the outer margin ol the 

 diluted ends becomes frilled as the filaments differentiate, 

 the dorsal, where the largest slits are, soon presenting fila- 

 ments connected only as their outer border, the free inner 

 edge being deeply grooved (bifid in section) (I'l. II. fig. 12). 

 The outer border of each filament has the tough cuticle 

 with the hypoderm beneath, in which is a nerve, and joining 

 in the centre a connective-tissue area which runs inward to 

 the free grooved edge, whilst the sides are strengthened by 

 the modified hypoderm, especially externally, for it tapers 

 internally. Each of the laminae forming the groove has a 

 blood- ve^^sel in its centre (PI. III. figs. 18 & 19), and 

 branches by-and-by enter the pinnules. Proceeding still 

 further distally, the curve in each fan is larger, and the 

 dorsal filaments, which have become rounder and their 

 hypoderm more glandular, show longer connecting bands, 

 and finally separate, the isolated ones having slightly 

 shallower grooves than the fixed, whilst their radial diameter 

 diminishes and their transverse increases proportionally. 



The filaments gradually taper .dia:t:ally, the edges of the 

 groove break into pinnae (PI. IV. fig. 25j, and the 

 modified hypoderm forms three distinct external divisions, 

 whilst in the centre is the connective-tissue area with its 

 blood-vessel^ a vessel occurring also in each pinna. Besides 

 the central blood-vessel there are two conspicuous channels 

 slightly to the exterior on each side, arid these probably 

 communicate with the coelom. In longitudinal sections of 

 the filaments the centre shows a distinctlv chordoid structure 



