176 PERCY SLADEN TRUST EXPEDITION 



distal ly in a rather long, slender tail. The proboscis is short and stout. In the centre of 

 the disc is placed the visceral nucleus. A side view (fig. 2) shows the fin, with rounded 

 edges, situated somewhat more distally than the visceral nucleus ; a sucker is visible 

 at the hinder margin, near the base of the fin. 



The animal measures from tip of the proboscis to end of the tail 12 mm., the breadth 

 of the disc is 6 mm. Lesson* gives for the length about 80 mm., for the breadth 40 mm. 

 This seems to be a quite extraordinary size, as Hedleyt notes 30 mm. and 13 mm. 

 respectively, the first measurement being taken from the eyes to the end of the tail. The 

 specimen of the Challenger, which I described J, was about as long as the animal now 

 under consideration. 



The disc, as has been said above, exhibits a heart-shaped form, slightly emarginated 

 at the anterior edge. The whole cutis, of which the disc is formed, consists of a gelatinous i 

 but tough, wholly transparent substance, which, on microscopical examination, proves 

 to originate from small branching cells, widely scattered within the gelatinous material, 

 thus forming a tissue such as is found in pelagic animals, and especially in such forms 

 as Carinaria, Pterotrachea, etc. Numerous nerve-threads and -cells are distinctly visible 

 in the cutis. The most conspicuous feature however consists in the differently shaped 

 opaque dots, which are distributed all over the surface of the cutis, also on the proboscis, 

 though lying beneath the outer epithelium. These dots have already been noted by 

 Lesson and Hedley ; I myself described them in Cardiapoda§ and in Carinaria macro- 

 rhynchus Tesch ; here in Pterosoma they exhibit essentially the same structure, so I have 

 thought it superfluous to figure them again. Whether these dots are glandular or not 

 seems to be uncertain ; I could never detect any efferent duct, and the constituent elements 

 of the agglomeration have not the aspect of unicellular glands. The close resemblance, 

 however, of these speckles with those described and figured by Vayssiere|| in Carinaria 

 pseudorugosa, called by the author "corpuscules glandulaires " and examined under a very 

 strong magnification, pleads for a similar nature in Pterosoma. At the margins and in the 

 middle the dots are small, very elongated and situated in a longitudinal direction, except 

 distally from the visceral nucleus, where they are placed transversely (fig. 1). Between the 

 two regions named above the dots assume a rounded shape and are a trifle larger. Here 

 and there, especially on the proximal half of the disc, some few tubercles are scattered 

 about. 



The great development of the cutis and the formation of a disc, eminently adapted to 

 a pelagic habit, renders powerful muscles superfluous. In fact the whole musculature on the 

 body has been diminished in the extreme, and is scarcely visible through the cutis. Near 

 the mouth, the usual muscles for moving the buccal mass and the radula are present, 

 in the same way as in Cardiapoda ; distally, on the proboscis, and on the body within the 

 cutis, only a very faint longitudinal musculature exists ; on the proboscis this is crossed by 

 a few transverse fibres. On the tail there are some rather strong longitudinal muscular 



* Voyage de la " Coquille," Zool, T. ii. p. 255, PI. ill. figs. 3, 3 bis. 



t I.e. p. 333. + I.e. p. 23. § I.e. pp. 73, 76, PL x. figs. 49, 52. 



|| Mollusques H^teropodes provenant des campagnes des yachts Hirondelle et Princesse Alice, Camp. 

 Sc. Albert J. th Monaco, Fasc. xxvi. p. 21, PI. vi. figs. 82—84, 1904. 



