156 THE MECHANICAL FUNCTIONS. 



made this selection, it there fixes itself unalter- 

 ably for the remaining term of its existence.* 



The parts of the Spongia jjanicea, which are 

 naturally transparent, contain at certain seasons 

 a multitude of opaque yellow spots, visible to 

 the naked eye, and which, when examined by 

 means of a microscope, are found to consist of 

 groups of ova, or more -properly gemmules,'\ since 

 we cannot discover that they are furnished with 

 any envelope. In the course of a few months 

 these gemmules enlarge in size, each assuming 

 an oval or pear-like shape, and are then seen 

 projecting from the sides of the internal canals 

 of the parent, to which they adhere by their 

 narrow extremities. In process of time, they 

 become detached, one after the other, and are 

 swept along by the currents of fluid, which are 

 rapidly passing out of the larger orifices. Fig. 

 55 represents one of these gemmules detached 



* Phenomena, which appear to bear some analogy with these, 

 have been noticed in the vegetable kingdom. The tribe of Zoo- 

 carpia, produce a kind of fruit, which when detached from the 

 parent, appears to possess powers of spontaneous motion, until 

 the period of its taking root, and growing like a vegetable struc- 

 ture. These singular productions, which seem, in their progres- 

 sive developements, to possess alternately the characters of 

 vegetables and of animals, may peihaps be regarded as connect- 

 ing links between the two great kingdoms of living nature. 



t Gemmule is a term derived from the Latin word gemma, a 

 bud : and its meaning, as applied to zoophytes, is that of a 

 young animal, not contained within an envelope, or ^^^'^. 



i 



