314 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



the discs in the interspace may by-and-by be seen grouping together, so as to 

 form stellate bodies, resembling miniature solasters, spiked bodies like thorn apples, 

 flattened structures with pectinate ends, and various other forms. In O. melan- 

 cephala the discs are comparatively small, some being clear, spindle-shaped bodies, 

 others granular and rounded. The enormous increase of cells and granular 

 masses in the proboscidian fluid, after the discarding of a proboscis, is well seen 

 in this species. In Tetrastemma the discs (Plate IV. fig. 14), though similar in 

 shape to those of 0. alba, are comparatively large ; and in a variety of T. raricolor, 

 which I am at present inclined to regard as the Polia sanguirubra of M. de 

 Quatrefages, they are tinged pinkish or reddish by transmitted light (Plate IV. 

 fig. 11). They are not all similarly tinted, some being pale, others yellowish, while 

 many are bright red — the colour in all cases being in the nuclei. Circular bodies 

 and granules are present, as in Ommatoplea. The skin of this specimen contained 

 many minute reddish pigment specks, so that to the naked eye it had a delicate 

 salmon-pink appearance. Reddish granular masses occasionally occur in the 

 proboscidian chamber of 0. alba, and in other species of Tetrastemma, generally 

 associated with reddish specks in the skin; and it is curious that a cast-off pro- 

 boscis in T. algos, and other species, assumes the same hue by transmitted light. 

 With the foregoing exceptions, the only changes noticed in the colour of the discs 

 were those caused by refraction of the rays of light. After extrusion into the 

 water, their shape soon alters, and they adhere together, and become translucent. 



M. GErsted* gives a small figure of a transverse section of his Notospermus 

 flaccidus, and characterises the proboscidian cavity as " canalis in quo penis est," 

 indicating by a blank beneath what might have been the digestive . tract. His 

 interpretation of structures, however, is more distinct in his section explana- 

 tory of the Family Amphiporina,f in which the digestive cavity is correctly 

 alluded to. 



The reflection of the walls of the proboscis before-mentioned, in front of the 

 ganglionic commissures, is the only barrier (and a very effectual one) I have 

 observed separating the proboscidian chamber from the tissues of the head. In no 

 species examined has such a cephalic diaphragm as described by M. de Quatre- 

 fages been found ; but the peculiar ciliated chamber or oesophagus, to be described 

 hereafter, takes its place, and leads one to infer that the distinguished naturalist 

 has misinterpreted the structure. Besides, the head is not a hollow organ, 

 requiring such definition from the other parts of the body. This author, while ex- 

 plaining a transverse section through Nemertes Borlasii\ (vel Borlasia Anglice), 

 shows a canal surrounding the proboscis ; but in his description he confounds it 

 with the general cavity of the body, and figures (fig. 5 same plate) the proboscis as 



* Entwnrf einer syst. Eintheilung, &c. der Plattwiirmer, tab. iii. fig. 51. 



f Entwurf &c, p. 94, fig. 18(woodcut). 



| Recherches Anat. and Zool. vol. ii. pi. xviii. fig. 4. 



