474 



POLYZOA. 



(1240). The embryo enclosed within the egg at first presents a 

 rounded form, but soon becomes divided by an indentation into an 

 anterior and posterior moiety, and vibratile cilia become apparent upon 

 the anterior extremity. That portion upon which the cilia have made 

 their appearance next insensibly enlarges, and assumes the shape of a 

 funnel, while the long cilia with which it is fringed begin to keep the 

 particles suspended in the water around in rapid motion. The margins 

 of the funnel rapidly extend themselves; the body exhibits frequent 

 contractions, and at the end of about two hours little tubercles are 

 apparent upon its anterior extremity, which subsequently become de- 

 veloped into the tentacula. Professor Yan Beneden thinks that when 

 the tentacula have become developed and furnished with their proper 

 vibratile apparatus, the original cilia disappear. The formation of the 

 tentacula at once indicates which are the two extremities of the body, 

 and the point by which the embryo will subsequently attach itself. 



(1241.) The embryo, when mature, is quite free, and strikingly re- 

 sembles some forms of Infusoria ; but after a while a pedicle is formed, 

 whereby it proceeds to fix itself to some foreign body, and thus per- 

 manently assumes the aspect of its race. The pedicle seems to be 

 formed from a cell, developed below the stomach, which grows directly 

 outwards, and thus completes the organization of the young Polyzoon. 



(1242.) A third form of reproduction is that by ciliated gemmules, 

 common in Halodactylus diaphanus and other similar species having soft 

 and fleshy or gelatinous poly- 

 paries. These are readily 

 seen in spring, when they 

 appear as minute whitish 

 points imbedded in the sub- 

 stance of the mass ; some- 

 times, however, they are of 

 a dark-brown colour, and 

 exceedingly numerous, ap- 

 pearing to occupy almost the 

 entire substance of the po- 

 lypary (fig. 240). If one of 

 these points bo carefully 

 turned out with a needle 

 and examined, it is found 

 to consist of a transparent 

 sac, in which are contained 

 generally from four to six 

 of the gemmules, which, as 

 soon as the sac is torn, 



escape, and swim about with the greatest vivacity*. 

 * Dr. A. Farrc, Phil. Trang. 1837, p. 140. 



Thin transverse section of Halodactylus diaphanus. 

 The centre occupied by cellular tissue and water ; the 

 circumference formed by cells in close approximation ; 

 the brown bodies scattered through the substance, 

 a a, Position of the gemmules enclosed in their sac ; 

 6, one of the gemmules escaped during the section of 

 the central tissue. 



Sometimes they 



