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



pore is furnished with a prominent papilla, covered with a fan-shaped brush of 

 cilia, the whole being only occasionally extruded, and no doubt assisting the 

 papillae previously mentioned in the tactile functions of the snout. This central 

 papilla is sometimes bilobed, and each of the divisions supplied with cilia. In 

 spirit-preparations of large examples of Linens longissimus the proboscidian 

 aperture is distinguished by a slight slit on the inferior surface immediately 

 behind the tip of the snout. 



Proboscis. — The proboscis (Plate X. fig. 1, a) commences as a somewhat 

 slender tube just in front of the commissures, gradually enlarges, continues for a 

 considerable distance of nearly equal calibre, and then, diminishing, terminates 

 posteriorly in a long muscular ribbon (\J/-, sometimes bifid), which, curving 

 forwards in the ordinary state of the parts, becomes attached to the wall of the 

 proboscidian tunnel. Its cavity is continued in front into the canal of the snout, 

 and posteriorly terminates in a cul-de-sac at the commencement of the muscular 

 ribbon. It differs from the Ommatoplean organ in certain respects, such as the 

 absence of the stylets, its more slender proportions, and the shape of the glandular 

 papillae on its internal surface. Experience, indeed, generally enables the observer 

 to distinguish by external characters the proboscis of a Borlasian from that of an 

 Ommatoplean in spirit-preparations, by the abrupt diminution of the calibre at 

 the posterior portion in the latter, caused by the presence of the stylet-region and 

 swollen reservoir ; but even where the organ is incomplete, a transverse section 

 at once puts the question beyond doubt. This was illustrated in a well-preserved 

 though shrunken fragmentary specimen brought by Mr Gwyn Jeffreys, the 

 distinguished conchologist, from North Unst, Shetland. At first sight it looked 

 like a Borlasian organ, on account of the absence of the stylet and posterior 

 regions, and from its large size I thought it Mould demonstrate the structure in 

 that family favourably, but a transverse section gave a true Ommatoplean 

 anatomy, with the characteristic beaded and other layers ; and an examination of 

 the animal itself at once confirmed its relationship. In the living animal the organ 

 is proportionally longer than in Ommatoplea, and when cast off becomes thrown 

 into numerous screw-like coils. Thus do the two great groups of soft worms 

 differ in essential characters ; and we are taught how unsafe is that classification, 

 e.g., such as Schmarda's,* which proceeds on other than anatomical grounds. 



A transverse section of the proboscis of a Borlasian (Micrura) from St Andrews 

 is represented in Plate XII. fig. 1. Externally there is a coat (a) similar to that 

 in Ommatoplea, apparently composed of homogeneous elastic tissue, yet showing 

 some granular markings towards its outer border. This coat is tougher than any 

 of the others, and often retains its integrity after they have ruptured. A powerful 

 longitudinal muscular layer (b) lies within the former, its cut fibres in transverse 



* Neue Turbel. Rotat. und Anneliden, vol. i. pt. 1, 1859. 



