72 



ESSAY ON CLASSIFICATION. 



Part I. 



proles medusina,^ and whether they separate or remain connected, then* structural 

 relations are everywhere the same. A comparison of Hydractinia, which is the 

 most common and the most polymorphous Hydroid, with our common Portuguese 

 Man-of-War (Physalia,) may at once show the homology of their most polymorphous 

 indi\dduals. 



The embryology of Mollusks has been very extensively investigated, and some 

 types of this branch are among the very best known in the animal kingdom. The 

 natural limits of the branch itself appear, however, somewhat doubtful. I hold that 

 it must include the Bryozoa,^ which lead gradually through the Brachiopods^ and 

 Tunicata to the ordinary Acephala, and I would add, that I have satisfied myself 

 of the propriety of uniting the Vorticellidoe with Bryozoa. On the other hand, the 

 Ceplialopods can never be separated from the Mollusks proper, as a distinct branch ; 

 the partial segmentation of their yolk no more afibrds a ground for their separation, 

 than the total segmentation of the yolk of Mammalm would justify their separation 

 from the other Vertebrata. Moreover, Ceplialopods are in aU the details of their 

 structure homologous with tlie other Mollusks. The Tunicata are particularly inter- 

 esting, inasmuch as the simple Ascidians have pedunculated young, which exhibit the 

 most striking resemblance to Boltenia, and form, at the same time, a connecting hnk 

 wdth the compound Ascidians.* The development of the Lamellibranchiata seems to 



^ I shall show this full}' in my seccaid volume. 

 Meanwhile, see my pai)er on the stiiicture and 

 homologies of Radiata, q. a., p. 20. 



^ Allman, (G. J.,) On the Present State of our 

 Knowledge of the Fresh Water Polyzoa, Proc. Brit. 

 Asso. Adv. Sc., 20th Meet., Edinburgh, 18.50, p. 305. 

 — Proc. Irish Ac. 1850, vol. 4, p. 470. — Ibid., 1853, 

 vol. 5, p. 11. — VanBeneden, (P. J.,) Recherches 

 sur I'Anatomie, la physiologic et le developpement 

 des Bryozoaires qui habitent la cote d'Ostende, Nouv. 

 Mem. Ac. Brux., 1845, vol. 18. — Dumoktier, (B.. C.,) 

 et VanBeneden, (P. J.,) Ilistoire naturelle des 

 Polypes composes d'eau douce, Mem. Ac. Brux., 

 1850, vol. 16, 4to. fig. — HiNCKS,. (Tu.,) Notes on 

 British Zoophites, with Descriptions of some New 

 Species, Ann. and Mag. Nat. Hist., 2d ser., 1851, 

 vol. 8, p.' 353. — EmiENBEiiG, (C. GL,) Die Infu- 

 sionsthiere als vollkommene Orgajiismen,, Leipzig, 

 1838, 2 vols. fol. fig. — Stein, (F.,) Infusionsthiere 

 auf ihre Entwickelungsgeschichte untersucht, Leip- 

 zig, 1854, 1 vol. 4to. fig. — Frantzius, (Al. v.,) 



Analecta ad Ophrydii versatilis historiam naturaleni, 

 Vratislav, 1849. — Lachmann, (C. F. J.,) Ueber die 

 Organization der Infusorien, besonders der Vorticel- 

 len, Miiller's Arch., 185C, p. 340. Having satisfied 

 myself that the Yorticellidse are Bryozoa, I would 

 also refer here ta all the works on Infusoria in which 

 these animals are considered. 



' I see from a short remark of Leuckart, Zeitscli. 

 f. wiss., Zool., vol. 7, suppl., p. 115, tliat he has also 

 perceived the close relationship which exists between 

 Brachiopods and Bryozoa. 



* Savigny, (,.J. C.,) Memoires sur les Ajiim. sans 

 Vertebres, etc. q. a. — Chamisso, (Ad. a.,) De 

 animalibus quibusdam e classe Vermium Linnoeana, 

 Fasc. 1, De Salpa, Berol, 1819, 4to., fig. — Meyen, 

 (F. .J.,) Bieitriige zur Zoologie, etc., 1st Abth., iiber 

 Salpen, Nov. Act. Nat. Cur. 1832, vol. IG. — 

 Edwards, (H. Milne,) Observations sur les Asci- 

 dies composees des cotes de la Manche, Paris, 1841, 

 4to., fig. — Sars, (M.,) Beskrivelser, q. a. — Fauna 

 litt., q. a. — VanBeneden, (P. J.,) Recherches sur 



