39 



o 



Some further information with regard to the arms of Cephalodiscus is given in describing 

 the muscular system (Sect. XII), while the tentacles are alluded to under the heading of the 

 vascular system (Sect. XIV). 



The collar c a v i t )' and its contents. 



The collar-cavity consists of a pair of coelomic .sacs which transversely encircle the oral 

 region of the animal. The dorsal mesentery persists throughout, while the ventral mesentery is 

 only represented in that region which lies posteriorly to the origin of the operculum (PI. IX, 

 fig. 107; PI. X, figs. 123, 124, v.mes.'). In front of this region the right and left cavities 

 become continuous by the disappearance of the mesentery (fig. 122), a result which probably 

 allows the operculum to behave as a single organ; — which it could hardly do if it were 

 subdivided by a median septum. 



A sagittal section of C. dodecalophus (PI. IV, fig. 42) shews a considerable extent of 

 collar-cavity on the dorsal side of the body, and a very small portion on the ventral side. The 

 operculum is short in the middle ventral line, and a section in this region accordingly shews 

 the minimum amount of collar-cavity which it is possible to see. A median sagittal section 

 should theoretically include the dorsal mesentery of the collar; but that membrane is extremely 

 thin, and moreover it seldoms lies completely in one plane (cf. PI. XI, fig. 137). Fig. 42 thus 

 shews one of the collar-cavities, instead of the mesentery, on the dorsal side. 



While the posterior boundary of the collar-cavity lies in a plane which is nearly transverse 

 to the first part of the alimentary canal, the anterior boundary has a more complicated course. 

 In the position which has been assumed by the individual shewn in fig. 42, the dorsal part of 

 the collar-cavity is elongated in a direction which makes rather more than a right angle with 

 the posterior wall. In other individuals, in which the neck is more sharply bent (as in figs. 150, 

 151), the dorsal part of the collar-cavity may lie almost in the same straight line with the 

 transverse part. The dorsal part of the collar-cavity, with which the arms are connected, is 

 however, so arranged as to overlie the proboscis-cavity on its dorsal side. A frontal section of the 

 buccal disc thus shews three cavities (PI. XII, fig. 152); the two collar-cavities {b.c.^ separated 

 by their dorsal mesentery [mes."), and the unpaired proboscis-cavity, which occupies the anterior 

 part of the section. Along the .septum Vi thus constituted runs the notochord {nc/i.) supported 

 medianly by the dorsal mesentery- of the collar, and laterall\- by the two halves of the septum '/;. 



M.\STERM.\N (03, p. 717) has stated that there is a difference between Cephalodisnis and 

 Balanoglossus in the relations of the notochord, which "lies in its primitive position in the 

 "collar, and is in no way produced into the pre-oral cavity. It is a backward ventral e.xtension 

 "of the buccal shield which makes the subneural gland [= notochord] lie in front of the mouth, — 

 "not, as in Balanoglossus, a forward median extension of the subneural gland into the 

 "pre-oral cavity". I confess that I am unable to appreciate this distinction. The notochord in 

 Balanoglossus may indeed project further into the anterior body-cavity than in Cephalodiscus^ 

 but I can see no morphological difference of importance between the condition shewn in the 

 figure on p. 23 and that which obtains in Balanoglossus. It is no doubt true that the notochord 

 is in relation with the two halves of the collar-cavity along practically its whole length 



