Affinities of the Onchidia. 263 



perhaps aberrant in certain points," and in process of becom- 

 ing an air-breather^. 



Against this conception of Onchidium as a Nudibranchiate, 

 comparative anatomy must, I think, enter an absokite protest. 

 From the outwardly superficially similar Doridfe these animals 

 are very widely separated; and, indeed, there is no single group 

 of that order, rich as it is in forms, to which the Onchidia 

 closely approach, or from which they could naturally be derived, 

 not even the Ascoglossa, with their varying nervous systems. 

 An alliance with, or derivation from, the Steganobranchiata 

 (Tectibranchiata) would certainly be much more possible. 

 Comparative anatomy^ however^ must quite decidedly claim the 

 Onchidia as Pulmonata, The examination of a large new 

 Onchidium {0. melanopneumon, Bgh.) from the Pacific (Fiji 

 Islands) , brought home by the ' Challenger ' expedition, as 

 well as that of other Onchidia [0. tonyanum, Q. & G., 0. 

 verruculatum, Cuv.), has taught me nothing elsef. 



The central nervous system of the Onchidia agrees with 

 none of the types occurring in the Nudibranchiata, at any 

 rate there is only a superficial resemblance to the Ascoglossa, 

 which are otherwise so distant (and derived from the Stegano- 

 branchiata). It is almost unintelligible how Ihering could 

 see here " exactly the same type of the nervous system that 

 is displayed by the ^olidise and Dorid^." On the con- 

 trary, the 9iervous system of the Peroniffi does not differ essen- 

 tially from that of the Pulmonata, In the latter, as is well 

 known, it consists of two superior cerebral ganglia, two inferior 

 pedal ganglia, and several (up to 5 or 6) ganglia placed more 

 or less unsymmetrically below the latter^ and belonging 

 chiefly to the visceral nervous system. The nervous system 

 of the Onchidia is also of this kind, only with the lowermost 

 part more condensed and reduced. 



The central nervous system { of Onchidium tonganum ap- 

 pears, when still enclosed in its sheath, as a broad ring, of 

 which the upper and lower arches are strongly flattened ; 



* Brock, he. cit. p. 872. 



t R. Bergli, " Keport ou the Nudibranchiata,'' m Reports on the 

 Scientific Results of the Exploring Expedition of H.M.S. ' Challenger,' 

 &c., vol. ix. 1884, pp. 12(3-150, pi. iv. tigs. 25-27, pi. v. figs. 1-27, pi. vi. 

 figs. 5-21, pi. vii. figs. 1-12, and pi. viii. fig. 14. 



X From want of material I was unable accurately to define the central 

 nervous system of the Onchidia formerly investigated by me (loc. cit 

 pp. 130, 141, 147), and the preceding investigations (see Ihering, /. c. 

 p. 230, Taf. iv. fig. 16) are scarcely of any use. I have therefore re- 

 sumed this investigation upon two large specimens of O. tonganum, Q. & 

 G., from the Nicobars. The relations of the ganglia were perfectly in 

 accordance in both individuals. 



