478 POLYZOA. 



(1256.) It is now generally understood that, wherever oviparous 

 reproduction occurs, there is a formation of spermatozoa ; and modern 

 observations have proved the existence of these distinguishing products 

 of the male sex in most genera of the ciliobrachiate polyps. Frequently 

 both the sexes are conjoined, so that there is a complete hermaphro- 

 ditism ; but in some cases the sexes are separate, and the number of 

 female individuals is greater than that of the males. Seeing that among 

 these compound animals the blood, or its representative fluid, is common 

 to an entire group, and that the ova as well as the spermatozoa are dif- 

 fused through this liquid before they are evacuated, a single male indi- 

 vidual may, strictly speaking, suffice for the fecundation of the eggs of 

 a whole colony. 



(1257.) But with regard to the ova themselves, a remarkable difference 

 is observable between those of the fluviatile and of the marine Polyzoa. 

 Among the Alcyonellce and other genera there exist two sorts of eggs 

 the one covered with vibratile cilia, capable of swimming freely about 

 exactly like Infusorial animalcules, and the other enclosed in a hard 

 shell, having somewhat the appearance of the seeds of some plants. 

 The first sort, without a shell, is also met with among marine species ; 

 but the second seems peculiar to the freshwater Polyzoa. In Crista- 

 tella mucedo, for example, the ova are of this latter description, being 

 enclosed in a dense horny shell, the exterior of which is covered over 

 with sharp booklets, giving them an appearance strikingly like some of 

 the Desmidieai (fig. 241, 2). This shell is probably intended to preserve 

 the ova during the winter season from being destroyed by the freezing 

 of the ponds in which they occur, while the marine polyps, being sub- 

 jected to no such changes of temperature, can dispense with such a 

 covering. It is on this account apparently that these ova are met 

 with sometimes naked, and sometimes provided with a shell ; and in 

 the same way, in the genus Paludicella (in which ova have not been 

 detected), the gemma? become invested on the approach of winter with 

 a horny covering. 



(1258.) As there are thus two modes of reproduction, so are there 

 two kinds of embryogenic development observable among the fluviatile 

 Polyzoa ; that is to say, the polyp which is produced from an egg is 

 formed in a different manner from that which is produced by the pro- 

 cess of gemmation. In the mature ovum, both the germinal spot and 

 the germinal vesicle are distinctly perceptible ; but in the nascent gemma 

 the existence of neither of these elements is to be detected. According 

 to the gemmiparous mode of propagation, the young individual is formed 

 by direct extension from the tissues of the parent. In the formation 

 of the embryo from an egg, there is, from the first, a complete isolation 

 of the newly-formed progeny : a vesicle or cell is formed, which, pre- 

 vious to its conversion into a new individual, requires the cooperation 

 of another cell, or, in other words, the ovum remains unproductive 



