326 ZOOLOGY 



SECT. 



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 zooicl 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- 

 Isemata, 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 

 represented in the Cambrian and later Palaeozoic formations by 

 forms which appear to have belonged mainly, if not exclusively, to 

 the Cyclostomata. In the later formations of the Mesozoic period 

 the Cheilostomata are also abundantly represented, and in the 

 Tertiary the latter sub-order greatly outnumbers the Cyclostomata. 

 The Tertiary Polyzoa flourished in certain localities in such 

 luxuriance that their remains form calcareous deposits of very 

 great extent, 



Sub-Class II. Endoprocta. 



While the sub-class of the Ectoprocta comprises a large number 

 of genera, that of the Endoprocta includes only Pedicellina (Fig. 260), 

 Loxosoma, and Urnatella, with one or two other less completely 

 known forms. They are all marine except Urnatella an Ameri- 

 can fresh-water genus. The feature indicated by the name 

 of the sub-class viz. the position of the anus within the 

 circlet of the tentacles, is an important point of difference from 

 the rest of the Class ; but there are others of as great or greater 

 importance. 



In none of the Endoprocta is there a distinct introvert. The 

 body is cup-shaped, with a rim which is capable of being inverted 

 over a cavity the vestibule within which the tentacles 'can be 

 withdrawn, and which contains both mouth and anus. An cpistome 



