46 



I am thus led to the coiulusioii tlial the proljlematical tissue must be inter[jreted as it 

 stands, without the aid of any hypothesis to the effect that rujnure has taken place; and it 

 remains to consider further the evidence afforded by the sections. Sagittal sections (fig. 140) 

 shew that the lamellae stand more or less at right angles to the wall of the collar-canal ; 

 or, in other words, that they radiate from its lunu;n. Frontal sections (fig. 121) shew that 

 they originate from the lateral body-wall ot the collar, an arrangement which is also seen in 

 transverse sections (fig. 131) or in the reconstruction shewn in fig. 24. Although it would not 

 be difficult to imagine that the lamellae had been torn away from the basement-membrane 

 covering the pharyn.x in figs. 120, 121, the fact that the oral muscle {or.m.) of the collar 

 intervenes between the pharynx and the problematical tissue makes it impossible to suppose 

 that this tissue was connected with tlie i)haryngeal wall along the whole of what appears as 

 its free border in figs. 1 20, 121. 



The relations of the problematical tissue can thus be expressed by saying that each of 

 the lamellae of which it is composed is connected, along about half of its circumference (see 

 fig. 131) with the thin dorsal wall of the collar-canal and with the external body-wall of the 

 collar, and that the remainder of its circumference is freely exposed to the collar-coelom. It is 

 difficult to see the purpose of this arrangement ; but I am inclined to think that the tissue must 

 be interpreted as muscular; that the principal mode of contraction of the lamellae is along the 

 line between the body-wall and the dorsal wall of the collar-canal ; and that the effect of the 

 contraction is to dilate the lumen of the canal and its aperture into the body-cavity. 



It is now necessary to return to figs. 47 — 49, in which fibres can be traced from the 

 dorsal wall of the collar-canal to the oral epidermis of the ojjerculum. These figures refer to 

 C. gracilis, and although the arrangement at first sight seems to differ from that which has 

 been examined in C. dodccaloplms and C. levitiseni, 1 think that the difference is more apparent 

 than real. Thus in other specimens of C. gracilis I have obtained evidence that the problematical 

 tissue is composed of lamellae connecting the wall of the collar-canal with the external body- 

 wall, an arrangement which is not precluded by hgs. 47 — 49. Moreover, in the other species, 

 there is evidence that certain fibres pass from the wall of the collar-canal to the oral wall of 

 the operculum. This is shewn, in C. dodecalophus, in fig. 151, where in addition to the main 

 mass of the problematical tissue (.r.) a few delicate fibres, similar to the shorter fibres which 

 traverse the collar-cavities in such numbers, pass from the wall oi the collar-canal or even from 

 the problematical tissue, to the oral wall of the operculum. A similar arrangement is seen, in 

 neuter individuals of C. sidogae, in figs. 77, 78; and it is thus probable that the dilatation of 

 the collar-canal may be partly effected by the contraction of these fibres. 



I feel that 1 am far from having understood the mechanism of the collar-canals in 

 Ceplialodiscus, but their structural arrangements .seem to me to indicate clearK' that muscular 

 action is an im])ortant factor in their mode ot acting. The thin dorsal ejjithclium of the canals, 

 and its varying position, suggest that the movements of this part are of im]H)rtance, and it is not 

 improbable that it may to some extent have a simple valvular action, controlled by the fluid 

 pressure of the contents of the collar-cavity. 



I have not noticed an)- differences of importance between the different species oi Cephalodiscus 



