648 LECTURE XXIV. 



In the pi-qicr Radiated series itself we discern the same principle : 

 the radiated type culminates in the Echinoderms ; but the most typical 

 forms, called emphatically star-fishes, are pedunculated in the em- 

 bryo-state, at least in one family, and so far manifest conformity 

 of organisation with the Polypes and the vast and almost extinct 

 tribes of the Pentacrinites, before acquiring their free and locomotive 

 maturity. 



It will be found when we enter upon the consideration of the 

 development of the Vertebrate embryo, that its unity of organisation 

 Avith the Invertebrata is restricted to as narrow and transitory a point 

 as that of the Articulate with the Molluscous series. Manifesting the 

 same monad-like properties of the germ, the fissiparous products 

 proceed to arrange and metamorphose themselves into a vermiform 

 apodal organism, distinguished from the corresponding stage of the 

 Insect by the Vertebrate characteristics of the nervous centres, — viz. 

 the spinal chord and its dorsal position ; whereby it is more justly 

 comparable to the apodal fish than to the worm. 



Thus every animal in the course of its development represents 

 some of the permanent forms of animals inferior to itself; but it does 

 not successively repeat them all, nor acquire the organisation of any 

 of the inferior forms which it transitorily typifies. Had the Animal 

 Kingdom constituted, as was once supposed, a single and continuous 

 chain of Being progressively ascending from the Monad to the Man, 

 unity of organisation might then have been demonstrated to the 

 extent in which the theory has been maintained by the disciples of 

 the school of Bonnet and GeofFroy St. Hilaire. 



There is only one organic form which is either permanently or 

 transitorily represented throughout the Animal Kingdom : it is that 

 of the microscopic infusorial Monad, with the consideration of wliich 

 the present survey of the Invertebrate animals was commenced, and 

 which is to be regarded as the fundamental or primary form. 



Other forms are represented less exclusively in the development 

 of the Animal Kingdom, and may be regarded as secondary forms. 

 These are, the Polype, the Worm, the Tunicary, and the Lamprey ; 

 they are secondary in relation to the Animal Kingdom at large, but 

 are pi'imary in respect of the primary divisions or provinces.* 



Thus the Radiata, after having passed through the monad-stage, 



* Von Baer adopted Cuvier's view of four leading types — Radiate or " peri- 

 pheral," Articulated or " longitudinal," Molluscous or " massive," and Vertebrate : 

 but he superadded the idea that the Vertebrate type united in itself the three 

 others, " the head being an outline of the radiate type." CLXXXIX. p. 74G. 

 Here, however, an unreal signification seems to be assigned to the " circulus 

 arteriosus " and its four vascular trunks. 



