646 LECTURE XXIV. 



through both the Infusorial and Polype stages, and propagates by 

 gemmation, as well as spontaneous fission, before it acquires its 

 mature form and sexual organs. The fulness of the unity of 

 organisation which prevails through the Polypes and larval Acale- 

 phes, is diminished as the latter approach maturity and assume their 

 special form. 



The Bryozoa, after simulating the higher Infusoria by their 

 spheroid shape and active movements, due to well-developed zones 

 or lobes of conspicuous vibratile cilia, mask their low molluscous 

 character beneath the polype form. The Ascidian Mollusks typify 

 more feebly and transiently the polype state in passing from that of 

 the cercariform ciliated larva to the more special molluscous form. The 

 Univalves and Bivalves obey the law of unity of organisation in the 

 spontaneous fissions of their amorphous germ, and in its ciliated epi- 

 thelium, by which it gyrates in the ovum ; but they proceed at once 

 to assume the molluscous type without taking on that of the Polype ; 

 the Bivalve retaining the acephalous condition, the Univalve as- 

 cending in its development to the acquisition of its appropriate head, 

 jaws, and organs of sense. 



Thus all mollusks are at one period like Monads, at another are 

 Acephalous; but few typify the Polypes, and none the Acalephes, 

 or Echinoderms. In the Encephalous division we meet with many 

 interesting examples of the prevalence of unity of organisation at 

 early periods, which is lost in the diversity of the special forms as 

 development proceeds. Thus the embryos of the various orders of 

 Gastropods are first abranchiate, next nudibranchiate, but only a 

 few retain that condition of the respiratory system through life; 

 most of them move at first by aliform anterior lobes, like those which 

 characterise the mature Pteropods, but afterwards exchange the 

 swimming organs for the repent disc which marks their class. The 

 naked Gastropods are at first univalve Mollusks, like the great bulk 

 of the class at all periods. The testaceous Cephalopods first 

 construct an unilocular shell, which is the common persistent form in 

 Gastropods, the Polythalamia afterwards superadd the characteristic 

 chambers and siphon. This simple fact would of itself have dis- 

 proved the theory of ' evolution,' if other observations of the phe- 

 nomena of development had not long since rendered that once 

 favourite doctrine untenable. 



Thus, as we trace the development of the Molluscous animal, we 

 find the application of the term unity of organisation progressively 

 narrowed as development advances : for whilst all Mollusks rtianifest, 

 at their earliest and most transitory period, a resemblance to the 

 lowest or monadiform zoophytes, only the lowest order of Mollusks in 



