viii PHYLUM MOLLUSCOIDA 353 



zooid has undergone degeneration, or through a special opening 

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

 ova pass out through the intertentacular tube. In Crisia&nd other 

 Cyclostomata each of the ripe ocecia 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 pro- 

 cesses arise, 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 larva may or may not be 

 provided with a digestive canal. 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. By a metamor- 

 phosis similar to that which has been described in the case of Bugula 

 (p. 344), a primary zooecium with a primary zooid is developed from 

 the previously free ciliated larva. 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. 280, stato) is observable in the PhylactolaBinata. The 

 statoblasts are internal buds formed from the funiculus and 

 enclosed in a chitinous shell ; they are set free eventually by the 

 death and decay of the parent colony, and in spring each gives 

 rise to a small zooid which fixes itself and develops into a 

 colony. 



Ethology and Distribution. None of the Ectoprocta are 

 parasites in the .strict sense of the term, but very many of them 

 live in intimate association with other organisms, often growing 

 over and through them so as to form with them one complex 

 structure. Certain genera are able by some means to excavate 

 minute burrows in the shells of bivalves. 



The majority of Ectoprocta are marine ; but all the Phylacto- 

 kernata, together with Paludicella of the Ctenostomata, are in- 

 habitants of fresh water. The fresh-water forms inhabit both 

 running and stagnant waters; they occur at all elevations and 

 are represented in all the great regions of the earth's surface. 

 The marine forms are most abundant at moderate depths ; 

 but representatives of the group have been dredged from as 

 great a depth as over 3,000 fathoms. In certain localities the 

 larger kinds grow in great luxuriance, so as to form miniature 

 forests. 



Geologically the Ectoprocta are a very ancient group, being 



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