THE VOYAGE OF H.M.S, CHALLENGER. 



are always united in the median line. The jaw of the Thecosomata is always 



formed of two quite distinct pieces. 



6. The radula of the Gymnosomata is provided with 

 long, lateral teeth, with large basal pieces ; the number 

 of the teeth in a transverse row generally increases with 

 age, until the adult state is reached, so that a know- 

 ledge of the radula of young specimens is of little use 

 for specific identification. The radula of the Thecosomata 

 has but one lateral tooth on each side, with a small 

 basal piece. 



In spite of the well-marked differences between the 

 two groups, there are, however, some forms about the 

 systematic position of which all zoologists do not agree, 

 some placing them among the Gymnosomata, and 

 others among the Thecosomata. Halopsyche [Eurybia), 

 for instance, has been placed by many among the 

 Thecosomata, because its organisation was not well 

 known ; but we shall see further on tbat it is indis- 

 putably a Gymnosomatous Pteropod. As for the 

 Cymbuliidse, even Fol ' cpiestions whether they have 

 not more affinities with the Gymnosomatous than 

 with the Thecosomatous Pteropoda. But they cannot 

 be placed among the former, because they possess all 

 the essential characters of the Thecosomata as given 

 in the foregoing table, and do not exhibit any of those 

 of the Gymnosomata. There is, therefore, nothing to 

 justify Tiberi's opinion that Tiedemannia (Gleba), should 

 be placed among the latter. 2 



PlG. 1. — Diagram of a Pneunwnoderma, seen 

 from the right side, showing the different 

 external parts of a Gymnosomatous Pteropod. 

 1, right hook-sac; 2, position of the jaw; 

 3, ventral papilla of the proboscis ; 4, pro- 

 boscis ; 5, right acetabuliferous buccal ap- 

 pendage ; 6, right buccal tentacle ; 7, head ; 



8, position of the right nuchal tentacle; 



9, right anterior lobe of the foot; 10, orifice 

 of the penis ; 11, posterior lobe of the foot ; 

 12, anus ; 13, right fin ; 14, genital orifice and 

 spermatic groove, hidden beneath the fin ; 

 15, lateral gill ; 16, lateral somatic crest of 

 the lateral gill ; 17, posterior gill (quad- 

 rangular crest) ; 18, radiating crest of the 

 posterior gill ; 19, fringes of the radiating 

 branchial crest. 



3. The Habits of the Gymnosomata. 



The Gymnosomatous Pteropoda do not form a numerous group of species, but they 

 are found in all the seas of the world, and sometimes in great abundance. They are 

 small animals, not more than an inch and a half long, and are carnivorous, often feeding 

 on their Thecosomatous allies. 



Naturalists are not agreed as to the position assumed by the Gymnosomata when 



1 Sur le diiveloppement des Pteropodes, Archives d. Zool. exper., ser. 1, t. iv. p. 173. 

 - Mollusques inarms d'ltalie, Ann. Soc. Malawi. Belg., t. siii. p. 77. 



