40 



(cf. PI. XI, fig. 138), but the relations which it eventually acquires with the anterior body-cavity 

 and the pericardium (PI. XIII, fig. iSi) do not differ essentially from those found in the 

 Enteropneusta. This is indeed admitted by Masterman in a later part of his paper (p. 722.). 



Sagittal sections of C. Icvinseni (PI. IV, figs. 34 — 36) and C. gracilis (PI. IV, fig. 37; 

 PI. V, figs. 43 — 51) shew an essentially similar condition ; but in both these species the ventral 

 part of the collar-cavity is better developed than in C. dodecalophus, although in all three cases 

 the operculum is lower in the middle line than it is more laterally. 



The relations of the dorsal collar-region are best illustrated by means of sections transverse 

 to the lone axis of the zooid. The series of sections of C. leviiiseni shewn in PI. X are from 



O 



an individual whose proboscis-stalk was turned more dorsally than in PI. IV, fig. 34, so that 

 their direction, as compared with that figure would be represented by a line joining the posterior 

 end of the central nervous system to the tip of the ojjerculum. 



Fig. 117, through the base of the notochord, thus cuts the posterior part of the 

 proboscis-stalk in such a way as to shew no part of the proboscis-cavity (cf. fig. 33). The 

 collar-cavities occupy the whole of the proboscis-stalk, extending laterally into the arms and 

 separated medianly by their dorsal mesentery, at the ventral end of which lies the notochord. 



In fig. 115, which is further forward, two portions of the proboscis-cavity [p. c}~) appear 

 on the ventral side of the collar-cavities, one of than in a detached portion of the proboscis- 

 stalk, due to a fold, doiibtless caused by muscular contraction. 



In fig. 114, this fold has opened into the niain part of b.c)^ and the notochord is now 

 supported by three membranes, in the way that has been described above. In fig. 113, the 

 ventral body-wall of the proboscis-stalk is about to beconie continuous with the buccal disc. 

 The section cuts the tip of the notochord, the lumen of which is here considerably dilated. 

 The notochord has moved up the collar-mesentery, so as practically to reach the central nervous 

 system, as described by Masterman (03), and it is overlapped by the posterior end of the 

 pericardium {J>er.), which separates the two collar-cavities from one another so that they are no 

 longer divided by a simple mesentery (see also PI. XI, fig. 138). The collar-cavities now rapidly 

 diminish in passing forwards. Immediately in front ot the bases of the first arms (fig. 112) they 

 are left as a pair of small cavities, the anterior dorsal horns of the collar-cavity {ó.c.-a.), distant 

 from one another, and separated from the pericardium by the proboscis-pores {p.p.), just in 

 front of which the collar-cavities terminate. 



The whole of this region of the collar-cavity is lined by a strong basement-membrane, 

 which in addition to the .stiffening function which it doubtless possesses gives origin to muscles 

 that traverse the proboscis-cavity. It seems to me probable that the anterior horns of the 

 collar-cavities may be functionally related with the proboscis-pores, closing them by the contraction 

 of the muscles which originate from them, aided, it may be, by a dilatation of the anterior 

 horns of the collar-cavities by the forcing of fluid into them from the rest of the cavity. It is, 

 however, clear that the horns are most important structures in prolonging the origin of the 

 system of muscles (PI. XII, figs. 147 — 149) which radiate through the proboscis-cavity. 



The relations of the anterior horns of the collar-cavities described in C. Icvinseni ai-e also 

 fouml in the other species, and have already been described, in C. dodecalophns, by M.\sïerman (03). 



