342 MONOECIOUS MOLLUSCA. 



of the tunic, where they continue to grow and develop up 

 to the period of their final expulsion. At first they are 

 globular, perfectly white and transparent, and a round aper- 

 ture can be distinguished ; when a little larger this globule 

 exhibits four reddish specks, the first visible sign of as many 

 embryos, which, as they enlarge, unite to form a chain par- 

 tially encircling the globule, and the ring is only completed 

 at maturity. It is then, in fact, a new Pyrosoma already 

 compounded of four animalcules, and about to assume an 

 independent existence. But as yet the component animal- 

 cules, you will observe, are only four in number, while the 

 adult Pyrosoma consists of some thousands of individuals ; 

 and the Botrylli and other compound Tunicata are equally 

 multitudinous in the mass, while their embryos show even 

 fewer numbers on their birth. Some authors would explain 

 this increase with increase of ages by supposing that the 

 few originals rapidly become pregnant, and deposit their 

 ova in the common envelope, where they are hatched to 

 become in their turn the parents of others that arrange them- 

 selves, in relation to each other, after the design imprinted 

 on each species by Him who called them into existence, and 

 determined probably by some peculiar conformation of the 

 primary ovum. It obviously follows, on this hypothesis, that 

 there are no prescribed limits to the growth of the compound 

 Mollusk ; so long as life endures and nutriment is supplied, 

 the propagation of the constituent individuals will go on, 

 and in a constantly increasing ratio, and to this increase of 

 the common mass the death of the whole is the only check. 

 But this is contrary to the fact. The number of animalcules 

 which composes a system in the Synoicum may consist of ten 

 but not of fifty ; a system of the Botryllus of thirty and 

 not of a hundred ; and although in certain kinds of Pyro- 

 soma the number appears to rise to several thousands, yet 

 even these great assemblages have their prescribed enumera- 

 tion which they cannot exceed. Savigny, therefore, main- 

 tains that the compound Mollusk, with all its component 

 animalcules, results alone from the gradual and successive 

 developement of the compound egg, which is consequently 

 supposed to include in one envelope just as many germs as 

 there are to be future individuals. These germs are deve- 

 loped in a fixed succession, the first deriving the materials 

 of their growth from their parent and from the ovum it- 

 self; and by the nutriment provided and supplied by these 

 after their birth the others are probably stimulated on to 

 evolution.* 



* Mem. sur les Anim. s. Vert. ii. 58, 59 ; 121-2, and 124. 



