viii PHYLUM MOLLUSCOIDA 325 



their development in certain dilatations of the colony (Fig. 255, 

 oosc.), and in many of the Gymnolsemata (Cheilostomata) these 

 ovicells or ocecia^ as they are termed, take on a very definite 

 shape. 



Reproduction and Development. As a general rule the 

 Ectoprocta are hermaphrodite. Both ovary and testis are derived 

 from the layer lining the coelome (parenchyma or coelomic 

 epithelium as the case may be), or from the funicular tissue. The 

 testis may be single or double. In some cases there is a spermi- 

 duct continuous with each; in most this is absent, and the 

 spermatidia, as in Bugula, or the mature sperms, become free in 

 the coelome. The ovary is very generally situated towards the 

 oral end or about the middle, the testis towards the base. The 

 mature ova escape into the ccelome, and in some forms become 

 impregnated there apparently by the spermatozoa of the same 

 individual. The development of the larva may take place in the 

 ccelome or a special diverticulum of it ; in the Cheilostomata the 

 fertilised ova pass into the ovicells ; in some cases, both among 

 the Phylactolasmata and the Gymnolaemata, they are received into 

 a sheath formed by the tentacles of an imperfectly-developed 

 zooid formed in a zocecium in which the original zooid had 

 undergone degeneration. 



In those cases in which the early stages of the development are 

 passed through in the body-cavity of the parent, the ciliated 

 embryos may either escape through the zooacial aperture after the 

 zooid has undergone degeneration, or through a special opening at 

 the base of the tentacles. In some the fertilised ova pass out 

 through the intertentacular tube. In Crisia and other Cyclo- 

 stomata each of the ripe ooecia is found to contain a large number 

 of embryos, developed from one ovum. The ovum in this genus 

 segments to form a mass of cells from which finger-like processes 

 are given off, the end of each of these becoming constricted off to 

 form an embryo. 



Segmentation is total and approximately equal. The form of the 

 free-swimming larva varies considerably, but in most there is a 

 circular band with very long cilia, the corona, which may represent 

 the tentacular crown of the adult ; this divides the surface into 

 two regions oral and aboral the mouth as a rule opening on 

 the former, and the anus on the latter. The aboral portion of the 

 body presents a ciliated retractile disc or calotte ; on the oral side is 

 the sucker by which the larva afterwards becomes fixed. In the 

 Cyclostomata the larva is barrel-shaped, with the mouth at one 

 end, and at the other a prominence corresponding to .the retractile 

 disc. In the Phylactolsemata the larva is in the form of a ciliated 

 hollow cyst from which the colony is formed by gemmation. A 

 special form of asexual multiplication by means of bodies termed 

 statoblasts (Fig. 258, stato) is observable in the Phylactolaemata. 



