vra PHYLUM MOLLUSCOIDA 345 



they may pass out through the intertentacular tube or may 

 accumulate in the cells of the brown body. 



In many Ectoprocta the colony bears a series of remarkable 

 appendages the avicularia which are of the nature of modified 

 zooids. In typical cases the avicularium has the bird's-head-like 

 form that has been already described in the case of Bugula ; 

 sometimes it is completely sessile. A second set of movable 

 appendages found in some forms are the vibracula ; these are 

 long tapering whip-like appendages which execute to-and-fro 

 movements. Intermediate forms between avicularia and vibracula 

 show that the latter are extreme modifications of the former. The 

 avicularia are frequently found to have seized in their jaws minute 

 Worms or Crustaceans, and it is probable that their function, as 

 well as that of the vibracula, is defensive ; in the case of the 

 Selenariidce, which form unattached colonies, it is said that the 

 movements of the vibracula subserve locomotion. 



The impregnated ova in many cases undergo the early stages of 

 their development in certain dilatations of the colony (Fig. 281, 

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

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



Reproduction and Development. In general the Ectoprocta 

 are hermaphrodite. Both ovary and testis are derived 

 from the layer lining the ccelome (parenchyma or ccelomic 

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

 testis may be single or double. The spermatidia, as in Bugula, or 

 the mature sperms, become free in the ccelome. 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 there become impregnated apparently by the 

 spermatozoa of the same individual. The development of the 

 larva may take place in the ccelome or in a special diverticulum of 

 it ; in the Cheilostomata the fertilised ova pass into the ovicells ; 

 in some cases, both among the Phylactolsernata and the 

 Gymnolsemata, they are received into a sheath formed by the 

 tentacles of an imperfectly-developed zooid formed in a zoocecium 

 in which the original zooid had undergone degeneration. 



In those cases in which the early stages of development are 

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

 embryos may either escape through the zocecial aperture after the 

 zooid has undergone degeneration, or through a special opening 

 formed for them in the wall of the zocecium. In some the fertilised 

 ova pass out through the intertentacular tube, which plays the part 

 of an oviduct. In Crisia and other Cyclostomata 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 arise, the end of each of these 

 becoming constricted off to form an embryo. 



