VARIOUS MODES OF REPRODUCTION. 471 



and the mouth of the cell thus opened, they, by their contraction, can 

 close it like a door. 



(1225.) The existence of nervous ganglia has been satisfactorily de- 

 tected in many genera of the Polyzoa : it consists of a nervous ganglion, 

 situated immediately above the oesophagus, from each side of which pro- 

 ceeds a nervous cord forming a collar around that tube, as well as other 

 filaments distributed to the muscular system. 



(1226.) No organ of special sensation has been discovered in any 

 animals of this class, either in their adult state, or during the earlier 

 periods of their development. 



(1227.) From what is known concerning the propagation of the 

 Polyzoa, it would appear that their reproduction is effected in several 

 different ways. 



(1228.) The most ordinary is by the development of gemmae, or buds, 

 that sprout from the parent stem in the branched species, or, as in the 

 Flustrce and Eschar ce, are derived from the sides of contiguous cells. 



(1229.) In Pedicellina Belgica, the phenomena attending the gemmi- 

 parous mode of reproduction are the following* : First, there sprouts 

 from the common stem of the Polyzoon, without any determinate situ- 

 ation, a minute tubercle, which is simply a prolongation from the stem 

 itself; this tubercle gradually extends outwards, becomes more promi- 

 nent, and soon swells into a vesicle, which is the first appearance of 

 the new individual. Up to this period the interior of the vesicle is 

 organized precisely in the same manner as the stem itself, of which it is 

 only an extension; but now a cellule becomes visible in its centre, 

 which forms the point of departure whence the development of the 

 embryo proceeds. 



(1230.) Around this primitive cell a series of other very small cellules 

 soon group themselves, which seem to constitute the parietes of the 

 primitive vesicle, or blastoderm, the original cell representing the vitel- 

 line cavity. The bud now enlarges ; and as its growth proceeds, the 

 internal tissue becomes thickened, so as to fill it completely ; subse- 

 quently an indentation becomes apparent on each side of the little 

 cavity, separating the embryo into two halves, the inferior of which will 

 form the stomach, the superior the intertentacular chamber. 



(1231.) In Layuncula repens, the reproductive gemmse sprout from 

 the creeping stems which connect the individual animals, appearing at 

 first as a slight prominence that soon expands into a rounded tubercle, 

 which is the commencement of a new cell. On close inspection, each 

 gemma is found to consist of a transparent envelope, that is, in fact, a 

 continuation of the general investment of the animal, lined throughout 

 with a soft membrane, having its inner surface studded with minute 



* VanBeneden, "Recherclies sur 1'Anatomie, la Physiologie, etla Developpeinent 

 des Bryozoaires qui habitent la Cote d'Ostende" (Bulletin de PAcad. Roy. de 

 Bruxelles, toni. xix.). * 



