Royal Society. 543 



Bay, specimens of Macgillivraya, Cheletropis, and a few other °-enera 

 of minute pelagic Gasteropoda, apparently undescribed, were 5 daily 

 taken in the towing-net, and afforded him an opportunity of more 

 precisely determining the mode of attachment of the ciliated arms 

 which he had at first presumed to be naked branchiae. 



In his former paper* it was stated, more particularly of Cheletropis 

 Huxleyi, that the gills were of two kinds, viz. "covered" and 

 "naked;" the former, corresponding to those of the pectinibran- 

 chiate Gasteropoda generally, he has never found to be absent in 

 any of the genera ; but from further observation of the so-called 

 naked gills, while the animals were alive in their native element, he is 

 disposed to think that they are chiefly employed for prehension, and 

 probably as auxiliary organs of natation. When these ciliated 

 appendages are fully extended, the line of cilia is perfectly straight, 

 so that the frilled border, noticed in the previous account, turns out 

 to be a character depending simply on the partial contraction of the 

 longitudinal muscular fibres, preparatory to complete retraction of 

 the organs. They have no connexion with the mantle, but encircle 

 the mouth together with the tentacula and eyes, and coalesce at their 

 bases like the segments of a deeply-cleft calyx. In the specimens of 

 Macgillivraya examined the arms were quite transparent, but marked 

 at irregular intervals with cross streaks of brownish purple. In the 

 extended state they were several times the length of the shell, and, 

 like the arms of a polype, they rolled themselves up when touched' 

 and started back into the shell with surprising rapidity. They ap- 

 peared also to be exquisitely sensitive, exhibiting short twitching 

 movements when minute particles suspended in the water came in 

 contact with them. 



In the specimens of Macgillivraya now referred to, the respiratory 

 siphon consisted of a process of the mantle converted into a tube by 

 the mere apposition of its borders without organic union ; it was 

 moreover much shorter than had been usually observed in previous 

 examples, and the author thinks that those now under consideration 

 may be a variety, if not a distinct species. 



In his former examinations of this tribe of Gasteropoda, the 

 author had never found more than four arms encircling the head, but 

 he has 6ince discovered six in a single genus with which he had been 

 long familiar by external characters. In this case the operculige- 

 rous lobe of the foot is quite cylindrical and of some length, bearing 

 the peculiar operculum on its truncated extremity with the clawed 

 process pointing to the left side. The sucker-disc is very small, and 

 presents an anterior and posterior lobe. The two tentacula bear 

 each an ocellus on the outer side near the base, and the ciliated 

 arms, in every respect save number, resemble those of Macgillivraya 

 and its congeners. The clawed operculum is developed from a 

 spiral nucleus situate near the internal thickened border; it seems 

 to be a weapon of defence, and is wielded with great dexterity by 

 the little animal, which makes skips and jerks by means of its com- 

 plex loot, after the manner of JSussu or iStrumlnis. 



The author notices another member of this diminutive tribe which 

 * J'hil. Mag. p. 297. 



