7 o8 THE ANIMAL KINGDOM. 



in others this appears to be the case with certain zooids in a given colony, 

 whilst the remaining zooids possess a ripe ovary or a ripe testis only. 

 It is possible, however, that a testis or ovary may be developed at some 

 other time in these cases. Both ovary and testis are produced by the 

 growth of cells derived from the funicular tissue or the endocyst. The 

 ovary is often placed at the upper end of the zooecium, the testis at the 

 lower. The ripe products are set free into the coelome, where the ovum 

 may be impregnated by the spermatozoa of the same individual. In two 

 species of the genera, Alcyonidium and Membranipora, the spermatozoa 

 have been seen to escape through a flask-shaped and ciliated inter- 

 tentacular organ placed on the anal aspect of the body, and opening both 

 externally and into the coelome. It has been found in female as well as 

 male zooids, and is perhaps to be considered as a nephridium. The 

 number of ova in the ovary is very variable (2 to 30). Some of the 

 ovarian cells form a follicle for the growing ovum. The ripe ova pass 

 into an Ooecium or marsupial chamber in Cheilostomata, where they are 

 probably impregnated by the spermatozoa of another zooid, and develope 

 into larvae which escape through the Ooecial aperture. In Farrella and 

 Hypophorella the ovum escapes by a special aperture at the base of the 

 tentacles. In some Phylactolaemata, e. g. Alcyonella^ and some Gymnolae- 

 mata, e.g. Valkeria^ a rudimentary tentacular and digestive system is 

 produced by budding in a zooecium in which the perfect organs have been 

 lost (infra). The ripe ovum passes into the tentacular sheath thus 

 formed, is fertilised and developed into a larva which escapes through the 

 opening of the sheath. In many instances, however, the ovum or the 

 larva must be set free solely by the death and decay of the parent. 

 A special individual a gonoecium, or a gonocyst is sometimes developed 

 in which ova are found. The former occurs in Crisia (Cyclostomata), 

 some Cheilostomata y and in Alcyonidium (Ctenostomata), the latter in 

 some Cyclostomata, but the anatomy of these structures is not understood. 

 The ovum undergoes total segmentation. In Phylactolaemata it forms 

 a hollow cyst ciliated externally, with walls formed of two layers of cells. 

 The colony is derived by gemmation from this cyst. There are three 

 forms of larva among Gymnolaemata, the bivalved larva of Flustrella 

 and of Membranipora, the latter known as Cyphonautes : the larva of 

 other Cheilostomata, and Ctenostomata, and that of Cyclostomata, The 

 Cheilostomatous and Ctenostomatous larva has an aboral face separated 

 by a ciliated ring or corona from an oral face. In both there is an aboral 

 calotte or retractile disc, surrounded by, or bearing sense-hairs, sometimes 

 regarded as the homologue of the foot-gland of the Entoprocta. On the 

 oral face are two structures : one the * anterior ectodermal furrow ' of 

 Vigelius, ' oral furrow ' of Nitsche and Claparede, the ' fente ' of Barrois, 

 the other, the ' sucker ' of Vigelius, ' stomach ' of Barrois, both produced 



