86 



MORPHOLOGICAL DEVELOPMENT. 



the Polyzoa in divers ways, and with different degrees of com- 

 pleteness. The little patches of minute cells, shown as magnified 

 in Fig. 153, so common on the fronds of sea-weeds and the 

 surfaces of rocks at low- water mark, display little beyond me- 

 chanical combination. The adjacent individuals, though sever- 

 ally originated by gemmation from the same germ, have but 

 little physiological dependence. In kindred kinds, however, 

 as shown in Figs. 154 and 155, one of which is a magnified 

 portion of the other, the integration is somewhat greater : 

 the co-operation of the united individuals being shown in 

 the production of those tubular branches which form their 



133 



common support, and establish among them a more decided 

 community of nutrition. 



Among the Ascidians, another order of the Molluscoida, this 

 general law of morphological composition is once more dis- 

 played. Each of these creatures subsists on the nutritive 

 particles contained in the water which it draws in through 

 one orifice and sends out through another ; and it may thus 

 subsist either alone, or in connexion with others that are 

 in some cases loosely aggregated and in other cases closely 

 aggregated. Fig. 156, Phallusia mentula, is one of the soli- 



tary forms. A type in which the individuals are united by a 

 stolon that gives origin to them by successive buds, is shown 

 in Percflwra, Fig. 157. Among the Botnjlliclce, of which one 



