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



refers under the head of O. pulchra :—" Immediately under the hearts" (ganglia) 

 " we observe a large, somewhat muscular, viscus, apparently hollow, and lying 

 in the course of the intestine, but seemingly unconnected with it. Of its office 

 and nature I can form no opinion ; but I may remark, that in all the species a 

 greater duskiness in its site shows that a similar organ exists in all." Prof. Kefer- 

 stein's notice* of the organ in (Erstedia pallida is very brief; and he has 

 abstained from figuring its relations, though affirming that its opening (constitut- 

 ing the mouth) is on the ventral surface behind the ganglia, as in the Borlasians. 

 M. Van Beneden,| while indicating an outline of the structure in Polia capitata, 

 makes no reference thereto in his descriptions. The same omission is made by 

 M. Claparede with regard to his figure of Prosorhochmus Claparedii, Kef. J 



In every specimen of Ommatoplea and Tetrastemma the great oesophageal 

 organ above-mentioned has been easily observed (Plate VIII. fig. 3, j) as an elon- 

 gated sac, slightly narrowed posteriorly, and usually thrown into various longi- 

 tudinal wrinkles. In ordinary views from above, it is seen to narrow somewhat 

 abruptly behind the ganglionic commissure, and to pass forwards beneath the 

 inferior one, to open at the tip of the snout just at its ventral border, as a short 

 longitudinal slit. I have seen the sac turned inside out here, and projecting 

 beyond the head in an animal which had been subjected to chloroform. Both 

 apertures may frequently be observed at once, — that for the proboscis being cir- 

 cular, while the mouth forms a short longitudinal slit beneath the former. The 

 observations on this point have been often repeated, out of deference to the dis- 

 tinguished foreign authors who hold different views, but I have never seen any 

 other aperture in the British Ommatopleans, and it were hard for such to exist 

 in the free portion of the oesophageal tube behind the ganglia. Moreover, as 

 shown in Plate IV. fig. 1, the narrow anterior part of the glandular oesophagus 

 lies close to the chamber for the proboscis, when the latter is in this region. The 

 two organs, proboscis and oesophagus, become more evidently separated from 

 each other in most sections, just in front of the ganglia, and the interposition of 

 the broad inferior commissure soon renders the distinction more evident ; there- 

 after they have the tunnel of the proboscis as a partywall, together with that 

 portion of the fibrous stroma of the extra-proboscidian region in which the median 

 blood-vessel is situated. The oesophagus, moreover, occupies a special chamber, 

 bounded by a series of well-marked fibres (Plate V. fig. 2, k), which pass down- 

 wards from the upper wall by the side of the proboscidian sheath, and unite in 

 the median line below it. The anterior narrow portion is generally translucent; 

 and just behind the commissure a pursed arrangement is often seen, which is 

 followed by the more opaque portion with its longitudinal rugae. The pursed 

 arrangement is very similar to that which is caused by tying the mouth of a 



* 



Op. cit. p. 70. t Mem. 1'Acad. Belgique, pi. iv. fig. 13. 



| Beobachtungen iiber, &c. pi. v. fig. 10. ph. 



