42 W. G. RIDEWOOD. 



(text-fig. 13, p. 38, mu. 4) is the "problematical tissue" of Harmer (10, pp. 42-46). 

 There is nothing in the present material to suggest that the structures in question 

 are " lamellae," and not fibres. The slope of the collar canal and the obliquity 

 of the muscle fibres to the antero-ventral wall of it are such that one's chances 

 of obtaining the fibres cut at right-angles to their length are very remote. The 

 ordinary sections that one takes for the study of the structure of Cephalodiscus are 

 those cut transversely to the long axis of the body, those taken parallel to the 

 sagittal plane, and those taken parallel to the ventral body-wall, i.e. parallel to the 

 face of the shield. All of these will cut the muscle fibres obliquely ; the chance 

 of getting some of the fibres cut longitudinally is greatest in the third kind of 

 section, but none of the three sections will show more than a few of the marginal 

 fibres cut transversely. The fibres are, further, not disposed strictly parallel with 

 one another, but they interlace slightly, so that in no case could all the fibres 

 of one section present the same aspect. 



As surmised by Harmer, who, in spite of what he considers to be its exceptional 

 shape, is disposed to regard the problematical tissue as contractile, the effect of the 

 contraction of the muscles is clearly to dilate the canal by drawing away the thin 

 antero-ventral wall from the stouter postero-dorsal wall. When the collar canal is 

 nearly closed, the transverse section of its cavity is crescentic ; when open, it is 

 elliptical, possibly nearly circular. 



The bodies in c|uestion are clearly those referred to as " solenocy tes " by 

 Schepotieff', and shown by him in his fig. 8 (29), and it is a curious coincidence that 

 Harmer at first suspected that they might be solenocytes, and even submitted his 

 preparations to Mr. E. S. Goodrich, in order to obtain the latter's opinion on the 

 matter (10, p. 45). They are obviously not solenocytes, however. What gives 

 them a faint resemblance to such excretory structures is the swelling which appears 

 at the coelomic extremity of each obliquely-cut fibre. This swelling, there seems to 

 be no reason to doubt, is due to the snapping of delicate fibres which in life must 

 have passed across the collar cavity to the opposite wall, these fibres being possibly 

 of a muscular nature themselves, like those that radiate across the proboscis cavity, 

 but they were certainly not so thick ; they must have resembled rather the ordinary 

 coelomic trabeculae. The resemblance to the latter is further borne out by the 

 presence of nuclei, exactly like those which are set upon the coelomic trabeculae, 

 occurring in numbers on the coelomic surface of the muscle. AVhat puzzles one at 

 first is the relation of the muscle fibres to other parts ; they have an origin, as 

 pointed out above, l»ut many, one might say most of them, have no insertion, the 

 fibres curving round, and ending apparently against the coelomic cavity only. 



The relations of the collar are prol)ably such that, during the death struggles, 

 when the animal is plunged into the killing fiuid, these muscle fibres in all cases 

 contract so violently as to break the trabecular connection extending from their 

 coelomic extremities to the opposite wall, i.e. the ventral face of the lateral flap of 



