CONCLUDING SUSIMAKY. 649 



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

 others proceed to be metamorphosed into the Acalephe or the Echi- 

 noderm. 



The Articulata, at an early stage of their development, assnme 

 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. 



The MoHusca pass from the condition of the ciliated Monad to that 

 of the shell-less Acephalan, and in like manner either remain to work 

 out the perfections of that stage, or diverge to achieve the develop- 

 ment of shells, of a head, of a pair of fins, of a ventral foot, or of 

 cephalic arms, with all the complexities of organisation which have 

 been demonstrated in the concluding Lectures of this Course. 



The Vertebrated germ having manifested its monadiform relations 

 by the spontaneous fission, growth, and multiplication of the primor- 

 dial nucleated cells, next assumes, by their metamorphosis and primary 

 arrangement, the form and condition of the finless cartilaginous fish, 

 from which fundamental form development radiates in as many and 

 diversified directions and extents, and attains more extraordinary 

 heights of complication and perfection than any of the lower secondary 

 types appear to be susceptible of. The ultimate stages of these de- 

 velopments, the various permanent or mature structures of the Verte- 

 brated series, Avith their physiological and other relations, will form 

 the subjects of succeeding lectures. 



