462 ZOOPHYTES. 



and its allies, besides tlie ordinary gemmation whicli extends the 

 original fabric, certain gemmse are developed, whicb gradually 

 come to present an organization altogether comparable to that of 

 the simpler Medusce (Fig. 222, b) and which, when detached, 

 swim freely away. These, there is good reason to believe, stand 

 in the same relation to the ordinary polype-buds, as the flower- 

 buds of a plant do to its leaf-buds; each medusa-bud containing 

 either male or female sexual organs, and performing its part in 

 the sexual act, after it has been set free from the polype struc- 

 ture that bore it, just as the male (or staminiferous) flower of the 

 Vallisneria spiralis discharges its pollen upon the female (or 

 pistilline) flower, whilst floating on the surface of the water, after 

 it has broken itself off from the stem. The ova thus fertilized, 

 being deposited by the free swimming Medusa-buds, evolve 

 themselves (it is probable) into single polypes, from eveiy one of 

 which there is gradually produced by continuous gemmation a 

 composite fabric, that in its turn developes Medusa-buds, whose 

 oflFspring resume the polype form. In Cordylophora, instead of 

 detached Medusa-buds, peculiar capsules sprout forth from the 

 stem, some of which contain sperm-cells, and others ova; and the 

 spermatozoa set free from the former enter the ovigerous capsules 

 and fertilize their ova, after the fashion of Vaucheria (§ 197). 

 The fertilized ova undergo " segmentation" according to the 

 ordinary type, the whole yolk-mass subdividing successively into 

 2, 4, 8, 16, 32 or more parts, until a " mulberry mass" is formed; 

 this then begins to elongate itself, the surface becoming smooth, 

 and showing a transparent margin ; and this surface becomes 

 covered with cilia, by whose agency these little bodies, now called 

 "gemmules," first move about within the capsule, and then swim 

 forth freely when liberated by the opening of its mouth. At this 

 period, the embryo can be made out to consist of an outer and 

 an inner layer of cells, with a hollow interior; after some little 

 time, the cilia disappear, one extremity becomes expanded into 

 a kind of disk by which it attaches itself to some fixed object; a 

 mouth is formed, and tentacula sprout forth around it; and the 

 body increases in length and thickness, so as gradually to acquire 

 the likeness of one of the parent polypes, after which the plant- 

 like structure characteristic of the genus is gradually evolved, by 

 the successive development of polype-buds from the first formed 

 polype and its subsequent oflPsets.' 



303. In the family Tubularidce, the long polype-stems are in- 

 vested by tubular horny sheaths; but these stop short below 

 the polype-heads, which are consequently unprotected ; and the 

 reproductive gemmae bud forth, as in the preceding case, from 

 the base of the tentacula. The most common form of this 

 family is the Tuhularia indivisa, which receives its specific name 

 from the infrequency with which branches are given off from 

 the stems, these for the most part standing erect and parallel, 



' AUman (loc. cit.). 



