370 LECTURE XXIV. 



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

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

 ganisation 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 

 with 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 typifies or 

 represents some of the permanent forms of animals inferior to itself; 

 but it does not represent all the inferior forms, nor acquire the 

 organization of any of the forms which it transitorily represents. 

 Had the animal kingdom formed, 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 Geoffroyan school. 



There is only one animal form which is either permanently or tran- 

 sitorily represented throughout the animal kingdom : it is that of 

 the infusorial Monad, with the consideration of which 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 primary in respect of the primary divisions or sub -kingdoms. 



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

 enter that of the polype ; many there find their final development ; 

 others proceed to be metamorphosed into the Acalephe or the 

 Echinoderm. 



All the Articulata, at an early stage of their development, assume 

 the form or condition of the apodal and acephalous worm ; some find 

 their mature development at that stage, as the parasitic Entozoa ; 

 others proceed to acquire annulations, a head, rudimental feet, 

 jointed feet, and finally wings : radiating in various directions 

 and degrees from the primary or fundamental form of their sub- 

 kingdom. 



