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



the terminal aperture the mouth, and indeed it could not be otherwise, since the 

 proboscis was regarded by them as the true alimentary organ. My observations, 

 while leading me to differ from M. Van Beneden and Professor Keferstein, 

 who aver that the Ommatoplean mouth is situated on the under surface behind 

 the ganglia, as in the Borlasians, coincide with the three former only in so far as 

 this anterior opening lies close to the real mouth (communicating with the ciliated 

 sac or oesophagus). Dr Max Schultze, almost alone amongst foreign authors, 

 seems to have noticed the true position of the mouth in his Tetrastemma obscurum. 

 The aperture for the proboscis lies just at the ventral border of the snout, while 

 the mouth forms a slit on the ventral surface immediately behind the former. 

 In this respect, therefore, there is a marked distinction between the Oramato- 

 plea and its allies on the one hand, and Borlasia and Cephalothrix on the other, 

 the mouth in the first group opening quite in front of the ganglia, while in the 

 other it is situated considerably behind the ganglia. Analogy gives no grounds 

 for supposing the proboscis to be the alimentary organ. 



I shall divide, for convenience of description, the Ommatoplean proboscis 

 into three regions, viz., the anterior, middle, and posterior. The first (Plate VI. 

 fig. 3, A) comprehends that somewhat cylindrical portion between the reflection in 

 front of the ganglionic commissures and the commencement of the stylet-region 

 — the trompe of M. de Quatrefages; the second (B) includes the stylet-region 

 proper and the well-marked swelling of the great muscular sac — the oesophagus 

 of M. de Quatrefages ; and the third (C) is represented by the long posterior 

 gland — the intestin of M. de Quatrefages. 



Anterior Region of Proboscis. — From the point of reflection backwards, the 

 proboscis {trompe, Riissel) gradually increases in diameter until its full size is 

 attained. The entire organ is proportionally on a larger scale than in Borlasia, 

 and its anatomy more apparent ; though I doubt, even in this group, if we can 

 assign it the ideal office of a vertebral column. The general appearance of the 

 commencement of the organ in 0. alba is seen in Plate VI. fig. 3, and in Tetra- 

 stemma algae, in Plate VIII. fig. 3. At the point of reflection there is sometimes seen 

 a kind of os, from the slight turning over of the lips of the organ in the early stage of 

 ejection (Plate VI. fig. 1, a). This figure also represents the longitudinal fibres of 

 the proboscis as most conspicuous in this region. Sometimes the organ assumes a 

 twisted position under examination, so as to give the fibres a spiral appearance, and 

 in such a state the structure might fancifully be likened to the spiral arrangement 

 of the muscular fibres in the oesophagus of the higher animals, but the condition is 

 purely accidental. I fear, however, it has led M. de Quatrefages into an erroneous 

 interpretation of the anatomy of the organ in Polia glauca* which (organ) 

 is described and figured as having regular spiral belts at its commencement. 



* Op. cit. plate xx. fig. 3. 



