262 Dr. R. Bergh on the 



the resemblance in the structure of the nervous system and 

 reproductive organs which the Onchidia present especially 

 to the basommatophorous Pulmonata is of quite a superficial 

 nature ; while he also ascribes no importance to various other 

 anatomical characters of the Onchidia wliich also occur in 

 the Pulmonata, either because they (such as the position of 

 the seminal duct) occur " only " in atypical Pulmonata 

 (Vaginalus), or because they (such as the position of the 

 typical eyes) have originated in another way than in the 

 Pulmonata *. With regard to the kidney, Brock is of 

 opinion that '' wherever we may begin with the phylogeny of 

 the Pulmonata, the kidney of the Onchidia is a true kidney, 

 and only in its adaptation to aerial respiration engaged in a 

 change of function, which, even if we regard the organ as 

 in course of becoming a lung, certainly has nothing to do 

 with the analogous adaptive phenomenon in the Pulmonata." 

 With regard to the affinities of these animals, however, the 

 developmental processes as displayed by Joyeux-Laffuie seem 

 to him to be quite decisive. Unfortunately this ontogeny, 

 which is so interesting on its own account, has for the 

 moment the less interest, because the tertium comparationis^ 

 or complete developmental history, embracing the first stages, 

 of one of the so-called Nudibranchiata, is, so to speak, com- 

 pletely wanting. Here, however, the strong development of 

 the velum in the larvaj of Onchidium must on no account 

 lead us to hasty conclusions ; Semper \ has already stated 

 that the larvae of " different species of the genera Auricula 

 and Scarabus, which belong to the Pulmonata, bear oper- 

 cula ; " and in the (still unpublished) sketches of the larvae 

 which Semper has sent to me the larva bears a large velum, 

 exactly like the Onchidium-lsiVYa, to which | Semper's figures 

 present a great resemblance §. The Pulmonate nature of 

 Scarabus can, however, hardly be doubted 1|. Ihering is 

 certainly, at least for the present, partially justified in express- 

 ing himself^ against the over-estimation of ontogeny in the 

 interpretation of phylogenetic relations and in classification, 

 and, in opposition to both Hackeland Semper, placing com- 

 parative anatomy in the foreground in the discussion of such 

 questions. From his whole revision of the above-mentioned 

 work Brock concludes that Onchidium is " a Nudibranchiate, 



* Brock, loc. cit. p. 372. 



t Semper, ' Die natiirliclien Existenzbedingiingeu der Thiere,' 1880, ii. 

 p. 101. English edition, p. 282. 



X See Joyeiix-Latiiiie, I. c, pi. xx. figs. 8, 9, pi. xxi. figs. 1-3. 



§ See also Ihering, Vergl. Anat. d. Nervensyst. 1877, pp. 203, 221. 



II Semper, loc. cit. i. 1880, p. 238. 



% Ihering, ' Ueber die syst. SteUung von Peronia,' 1877, pp. 37, 38. 



