SPONGES. 119 



The parts of the Spongia panicea, which are naturally 

 transparent, contain at certain seasons a multitude of opaque 

 yellow spots, visible to the naked eye, and which, when ex- 

 ^ amined by means of a microscope, are found to consist of 

 groups of ova, or more properly gemmules,^ since we can- 

 not 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 ra- 

 pidly passing out of the larger orifices. Fig. 55, represents 

 one of these gemmules detached from the parent sponge. 

 When thus set at liberty, they do not sink by their gravity 

 to the bottom of the water, as would have happened had 

 they been devoid of life; but they continue to swim by their 

 own spontaneous motions, for two or three days after their 

 separation from the parent. In their progression through 

 the fluid they are observed always to carry their rounded 

 broad extremity forwards. On examining this part with the 

 microscope, we find that it is covered with short fila- 

 ments, or cilia, which are in constant and rapid vibration. 

 These cilia are spread over about two-thirds of the surface 

 of the body, leaving the narrower portion, which has a whiter 

 and more pellucid appearance, uncovered. They are very 

 minute transparent filaments, broadest at their base, and ta- 

 pering to invisible points at their extremities: they strike 



noticed in the veg-etable kingdom. The tribe of Zoocarpia, 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 structure. These singular productions, which seem, in their 

 progressive developments, to possess alternately the characters of vegetables 

 and of animals, may perhaps be regarded as connecting links between the 

 two great kingdoms of living nature. 



f Gemmule is a term derived from the Latin word geinma, a bud: and its 

 meaning, as apphed to zoophytes, is that of a young animal, not contained 

 within an envelope, or egg. 



