REPORT ON THE PTEROPODA. 13 



The body of every Gymnosomatous larva possesses three ciliated rings, of which the 

 anterior is not continuous (see PL I. fig. 5) and is the first to disappear. Formerly, 

 before the relation of these larvae to the adult was known, they were considered and 

 described as distinct genera. 



These larvae show many resemblances to one another, and when they are merely 

 described without the complete development being traced out, or when the description 

 omits the characters of the foot, buccal appendages and radula, which alone allow of their 

 certain classification, it is very difficult to assert whether they belong to the Pneumono- 

 dermatidae or not. or at all events to say to which genus of this family they do belong. 

 It is not possible to determine specific distinctions from the radula alone, because the 

 number of lateral teeth increases with age, until adult life. 



We are therefore of course unable to assert with absolute certainty whether the 

 Trichocycli are larvae of the Pneunionodermatidae or of another family. It is, however, 

 probable that Trichocyclus dumerili, Esch., 1 is the larva of Spongiobranchsea australis, 

 on account of the length of its tentacles, which are much longer than those of Pneumono- 

 derma. I shall show further on that Trichocyclus mediterraneus, Costa, 2 is the larva of 

 Clionopsis Jcrohni. 



Trizonius csecus, Busch., 3 of Malaga, appears to belong to the present family, and to 

 the genus Pueumonoderma ; but it is very difficult to prove whether it really belongs to 

 this genus, because the published description of it, as well as those of the two Trichocycli, 

 are insufficient. 



The Mediterranean Gymnosomata, moreover, are not yet all known. Larvae formerly 

 described by Gegenbaur, 4 do not appear to belong to any known genus inhabiting that 

 sea, and ought to be placed in a new genus, Thliptodon, Boas, 5 the adults of which are 

 still unknown. The larva that Professor H. N. Moseley discovered, 6 belongs to the genus 

 Dexiobranchasa, as will be seen later on. 



1 Oken, Isis, p. 735, pi. v. fig. 4, 1825. 



2 Osservazioni su taluni Pteropotli del Mediterraneo, Annuario del Museo zoologico delta R. Universita di Napoli, 

 Anno v., p. 46, pi. i. fig. 3, 1865. 



3 Beobachtimgen fiber Anatomie und Entwicklung, &c, p. 112, pi. viii. fig. 10. 



4 Untersuehungen fiber Pteropoden und Heteropoden, pp. 95-97, pi. v. figs. 14, 15. 

 6 Spolia atlantica, p. 174. 



* On Stylochus pelagicus, &c, Quart. Journ. Micr. Sci., p. 32, pi. iii. fig. 14, 1877. 



