BRYOZOA. 153 



it in rapid circles, carrying every loose particle after them; or 

 they creep along the bottom of the watch glass upon one end with 

 a waddling gait: but at the expiration of forty-eight hours they 

 attach themselves to the surface of the glass, and the rudiments 

 of the cell may be observed. 



In the Flustra carbesia the ova are developed between the cell and 

 the body of the polype, which yields to, and is destroyed by, them as 

 they are developed. They produce ovate, yellow, ciliated larvce, 

 which, by virtue of the contractility of their tissue and the active 

 vibration of their cilia, escape with their small end foremost from 

 the opening of the cell. They then, after a short term of locomotive 

 life, settle and subside, the outline of the cell being first formed, and 

 the polype with its tentacula, muscles, and alimentary canal being 

 afterwards developed in a distinct small closed sac* The embryo 

 acquires its ciliated surface and power of contraction before it 

 ruptures the chorion ; it afterwards escapes from the parent cell. 

 These ciliated larvce are called " ova" in cviii. and other writings 

 of the same Author. Nordmann found in a Flustra of the species 

 called Tendra zostericola, that spermatozoa were developed in the 

 polypes of certain cells and ova in those of others, and that the 

 male cell communicated by an opening with a contiguous female 

 cell : this latter is distinguished externally by the trellis-work 

 pattern of its upper surface, that in the male being smooth. The 

 formation of the spermatozoa is ascribed to eight vermiform organs, 

 which are wanting in the female polypes ; but are attached near 

 the base of the tentacula in the males. 



There are a few genera of fresh -water polypes, e. g. Alcyonella, 

 Plumatella, and Cristatella, which have the ciliated tentacula in the 

 form of crescentic or horse-shoe lobes. The generative organ is a 

 membranous band extending from the bend of the stomach to the 

 bottom of the alimentary sac, in which are developed, according 

 to the sex, either ova or spermatozoa : the latter in sperm-cells which 

 succeed each other in a beaded series. The ova are usually few in 

 number : the germinal vesicle disappears, as in the ova of the Ascaris, 

 before the chorion is formed. This membrane is coriaceous, smooth, 

 and brown-coloured in the elliptic compressed eggs of Alcyonella, 

 The CristateUa has been observed to produce ova of a flattened 

 discoid form, with their outer surface singularly beset with long 

 bifurcated hooks like the infusorial Xanthidia. Tlie young Crista- 

 teUa undergoes its metamorphosis from the ciliated gemmule-state to 

 the mature form of the polype in the ovum, from which it escapes by 



* CVIII. pp. 116-118. 



