31 THE VOYAGE OF H.M.S. CHALLENGER. 



Pelagonemertes further, in that a series of polygonal areas are marked out on its surface. 

 The spirally wound organ, described as a tube, which is indicated in the figure of 

 Pterosoma, can scarcely be anything else than the proboscis of a Nemertine, the mouth, 

 at the extreme end of the body, being probably the aperture of the proboscis-sac, and the 

 fusiform nucleus the sac itself. On the other hand, it is difficult to conceive that Lesson, 

 with a number of sj)ecimens available for examination, could have missed seeing the very 

 conspicuously burnt-sienna-coloured ramified intestine of Pelagonemertes had such been 

 present in his Pterosoma. Further, in Pterosoma, a pair of elongate, closely opposed 

 eyes are described and figured, having transparent coloured cornese. 



" On the whole, now that a pelagic Nemertine is known to exist, there seems little 

 doubt that the animal seen and figured by Lesson was a Nemertine and not a Mollusc, 

 but it seems to have been a distinct form, with a pair of eyes and an unbranched digestive 

 tract." 



For the sake of facilitating reference to Lesson's two figures of Pterosoma, in which 

 Moseley is so strongly inclined to see a second species of pelagic Nemertine from the 

 tropical seas — a supposition in which I entirely concur — I have reproduced these on 

 PL VIII. figs. 1, 2. 



The single available specimen of Pelagonemertes, when it came into my hands in 

 September 18S4, was no longer entire, but consisted of two fragments of the body, and 

 of three small fragments of the proboscis. Both body-fragments were slit open on the 

 ventral side, the internal surface of the digestive tract, and partly that of the proboscidian 

 sheath, having thus been laid bare. The terminal portion of the body was missing 

 (vide Moseley, supra). My friend Professor MTntosh, who made these incisions and 

 examined the specimen before it came to me to be sectionised, has made the following 

 notes: — 



" The structure of the cutis corresponds with that of other fonns. In the preparation 

 the transparent gelatinous basis-substance for the most part alone remained, the granular 

 cells and clear mucous or gelatinous contents of these spaces having escaped. Beneath 

 this layer is a remarkable deep investment of basement tissue, which forms an elastic 

 investment all round the body. Numerous ducts, often having a zig-zag appearance, 

 pass through this coat. 



"The muscular layers of the body-wall, as Professor Moseley observes, are compara- 

 tively thin. The external circular fibres are hardly to be distinguished in ordinary 

 transverse sections, and form a thin layer outside the longitudinal. Such a condition 

 contrasts strongly with the well-marked circular coat in Amphiporus lactijioreus. The 

 longitudinal muscular coat is likewise comparatively thin, though it is better developed 

 than the circular. 



" Proboscis. — In the preparation the proboscis is partly extruded, and issues in the 

 same manner as in the ordinary form. It is streaked by longitudinal lines externally. 



