VI PEEFACE 



cation. The Spirotrieha are characterised by all possessing the adoral " heterotrichal " band 

 of large cilia ; they are divided into the sub-classes Heterotricha, Hypotricha, Peritricha, 

 and Oligotricha. The two first of these groups correspond with Stein's groups of the same 

 names, whilst the Peritricha of Stein are now divided into Peritricha and Oligotricha, the 

 latter sub-class being formed for such genera as Halteria, Strombidium, and Tintinnus. I 

 consider Butschli's classification an improvement upon Stein's, with the doubtful exception 

 of the distinct position assigned to the Oligotricha. 



In regard to the Hydrozoa, the most important additions to knowledge since the date 

 of the article are to be found in the large and richly-illustrated monographs by Haeckel 

 {System der Medusen, Jena, 1879-1880 ; "Eeport on the Deep-Sea Medusae," Challenger 

 Reports, vol. iv., 1882; "Reports on the Deep-Sea Siphonophora," Challenger Reports, 

 vol. xxviii., 1888), and in the remarkable researches of Weissman on the origin of the 

 sexual products {Enstehung der Sexualzellen hei der Hydromedusen, Jena, 1883). The 

 student who takes in hand the actual examination of a specimen of Aurelia aurita by 

 aid of the description given of it in the article Hydrozoa, should also refer to the plates of 

 Ehrenberg's account of this animal {Physikalische Abhandlungen der Konigl. Ahad. d. 

 Wissensch., Berlin, 1835), and Mr Minchin's brief but valuable paper on the enclosure of 

 the embryos in minute brood pouches formed by sacculation of the grooves of the oral 

 lobes {Proc. Zool. Sac, 1889, No. xxxix.). 



If I were rewriting the article MoUusca, I should adopt the conclusion of my friend 

 and former pupil, Dr Paul Pelseneer, of Ghent, and remove the Pteropoda from association 

 with the Cephalopoda, not to maintain them as a distinct .class, but to place them, as he 

 has done, among the Palliate or Tectibranchiate Opisthobranchiate Gastropoda, to which, it 

 seems, they bear the same relation as do the Natantia to the Azygobranchiate Streptoneura. 

 It appears that the Thecosomate Pteropods are nearly related to the BuUidse and Torna- 

 tellidse, whilst the Gymnosomate forms are derivable from the Aplysiidse. A careful study 

 of the nervous system convinced Dr Pelseneer that the sucker-bearing lobes of such 

 Gymnosomate Pteropods as Pneumodermon are really cephalic in nature, and innervated 

 from the cerebral ganglion, whilst the sucker-bearing lobes of the Cephalopoda are produc- 

 tions of the foot, and are convincingly demonstrated by Pelseneer (as maintained by me in 

 the article " Mollusca ") to be innervated by the pedal ganglia. The remarkable coincidence 

 in the Pteropoda and Cephalopoda of adoral appendages provided with suckers which had 

 been, to my mind, the chief ground for supposing' a genetic relationship between these 

 two sets of forms, proves to be a case of homoplasy.^ It is, indeed, a very striking case of 

 the parallelism of genetically distinct organs. The whole of this question is ably treated 

 by Pelseneer in Part III. of his " Report on the Pteropoda," published in vol. xxiii. of the 

 Challenger Reports, 1888. The student of molluscan anatomy should not fail to read this 



' The reader is referred for an explanation of this term, and a discussion of the phenomena concerned, to my article 

 " On the use of the term Homology in Modern Zoology, and the distinction between Homogenetic and Homoplastic 

 Agreements," Awn. and Mag. Nat. Hist, 1870. 



