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



Family V. Halopsychid^. 



1S50. Cymbuliadse, Gray {pars), Catalogue of the Mollusca in the Collection of the British 



Museum, pt. iL, Pteropoda, p. 24. 

 1856. Hyaleidx, Woodward (jiars), A Manual of the Mollusca, p. 204. 

 1859. Eurybidse, Chenu, Manuel de Conchyliologie, t. i. p. 115. 



Characters. — One pair of long flattened buccal appendages, not retractile into the 

 front part of the digestive tract. Proboscis absent. Gill absent. A bifid jaw. Unpig- 

 mented skin. 



Description. — Body ovate, of medium length, wide and thick, rounded behind. 

 Visceral mass extending to the posterior part of the body. The integuments are hard, 

 but do not form a shell at all. The head and fins of the contracted specimens may be 

 retracted into the body-wall, as in Pneumonoderma ; in that state, a dorsal extension of 

 the integuments of the body covers and protects the head. 



Head very small relatively to the body. 



Foot. — Anterior lobes free and rounded behind ; posterior lobe of moderate length, 

 contracted posteriorly. The separation of the posterior lobe of the foot from the fins is 

 more difficult to make out than in the other Gymnosomata, although the disposition is 

 the same, and the figures of Souleyet 1 and Huxley 2 are inexact on that point. 



Fins. — They differ from those of all other Gymnosomata. In the latter, they are always 

 abruptly contracted at the base, and their greatest width is placed immediately behind 

 this contracted part ; but the fins of the Halopsychidse on the contrary are very long, and 

 gradually narrow from the extremity towards the base, so that the greatest width is at 

 the distal end. This extremity is nearly truncated and slightly sinuous in the middle. 



Proboscis and Hook-sacs absent. 



Jaw. — The two halves extend dorsally, on eaeh side of the buccal orifice, forming a 

 long double or triple row of small toothed horny plates. 



Radula always with a median tooth, without denticles ; resembling that of Clione. 



Cephalic Appendages. — They have been described under various names, and generally 

 in an inexact and incomplete manner, by the zoologists who have studied the organisation 

 of Halopsyche. 



In this genus, as well as in all the Gymuosomata (with the exception of Clionopsis), 

 there are : — 



1. Tentacles properly so called. — As in all the other genera of Gymnosomatous 

 Pteropods, they number two pairs ; (l) the anterior, oral or labial tentacles, placed more 

 dorsally than in the other Gymnosomata ; they were called by most of the writers, " the 

 median branch inserted at the base of the tentacles ; " (2) the posterior, nuchal or 

 cervical tentacles, which have not yet been seen by any naturalist, except Huxley, who 

 calls them " rudiments of eyes upon the outer side of the base of the tentacles," 3 whereas 



1 Voyage de la Bonite, Zoologie, pi. xv. fig. 1. s On the Morphology of the Cephalous Mollusca, pi. iv. fig. 3. 



3 On the Morphology of the Cephalous Mollusca, Phil. Trans., p. 41, pi. iv. fig. 3, i, 1S53. 



