skeleton is composed in animals not having bone *,' as the shells 

 and their opercula f in Mollusks ; the calcareous crusts and 

 horn-like coverings of the Articulate animals ; the corals and 

 madraporic secretions of Zoophytes, for the defence and support 

 of their delicate gelatinous organs, cannot be regarded as 

 ' Peculiarities/ but as essential members of the normal system 

 of organs of support, equivalent to the bones of the vertebrate 

 animals. The subseries numbered 116. 139. and 140 in the 

 synopsis, have, therefore, been removed from the series of pecu- 

 liarities, and made to precede the parts of the skeletons of the 

 Vertebrata in the first subdivision of the present arrangement. 



Changes in the second division of the physiological collection 

 have been made in conformity with the same principles that 

 have regulated the alterations from the arrangement of the 

 synopsis, already noticed ; and chiefly consist in the reduction of 

 the formerly extensive series of ' Foetal peculiarities.' For as 

 every condition that characterizes the progress of the germ to its 

 extrication from the foetal coverings, or which disappears during 

 that progress, or is suppressed after birth, may be termed a 

 ' fcetal peculiarity,' numerous preparations had been transferred 

 from the older series illustrative of the phenomena of foetal de- 

 velopment in different classes of animals, and had been brought 

 together in the twentieth series of the arrangement of 1818, in 

 contravention of the special purpose of such older series. Certain 

 stages, for example, in the formation of the vitelline sac, exhi- 

 biting as many modifications of its relation to the embryo, were 

 shown in one series of preparations $ ; other stages were exhi- 

 bited in another series ; and a later condition, under the title 

 of 'Yolk received into the stomach ||,' formed a third separate 

 group. All such specimens have been brought together in the 

 present arrangement and placed in a consecutively ascending 

 order, subdivided only according to the class of animals, the 

 evolution of which such preparations successively illustrate. 



* Subserieei 116. t Subaeries 140. 



t Series XVIII., Subseries 193. Series XX., Snideries 208. 



|| Series XX., Subseries Sill. 



