976 



ZOOLOGICAL SOCIETY BULLETIN. 



head of the worm. The fluke worms are pro- 

 vided with one or more suckers by which they 

 retain their position upon or within the body 

 of the host. Some of these worms apparently 

 do not change their position once they have lo- 

 cated themselves, but others move about more 

 or less. Many marine worms live permanently 

 in tubes of calcareous or other matter, cemented 

 to shells, rocks, etc. 



Leeches also anchor themselves when feeding 

 by means of suckers, but these parasites affix 

 themselves only temporarily. The highly modi- 

 fled myzostcmas, parasitic on crinoids and star- 

 fishes, retain their hold by hooks on the appen- 

 dages. Some of these external parasites move 

 about over the host, while others live in a sessile 

 condition and stimulate the tissues of the host 

 to the formation of a sort of cyst somewhat like 

 a plant gall. One species is an intestinal para- 

 site. 



Bryozoa. — These minute animals are attached 

 throughout life, except in the larval stage. The 

 free-swimming larva possesses an adhesive 

 gland by which it fastens itself. It then un- 

 dergoes a metamorphosis, which is followed by 

 the usual colonial development in all except one 

 small primitive group (genus Lo.rosoma), in 

 which the separate individuals are fastened by 

 a cement from the pedal glands. The colonial 

 species, which are very numerous, have evolved 

 various methods for attachment of the colonies. 

 In all cases there is a secretion of chitin form- 

 ing an ectocyst, or outer wall of the body. This, 

 in many cases, is further strengthened by a 

 secretion of lime, giving firmer attachment. The 

 colony is spread in a layer over the substratum, 

 or portions of the colony may rise free from 

 one or more basal cells. In the case of the 

 species which have a single basal cell, the at- 

 tachment is usually made more secure by radical 

 fibres, sometimes very numerous, which arise 

 from other cells of the colony, and assist in hold- 

 ing it in position. In not a few instances the 

 colony spreads by means of a rhizome, occa- 

 sionally burrowing partially or entirely within 

 the substratum. In one case, the fresh- water 

 Cristatella, the colony is able to move about 

 slowly. 



Brachiopods: As a rule the members of this 

 group are attached by means of a pedicle or 

 stalk passing between the valves of the shell 



BRYOZOA ENCRUSTING A PEBBLE 



(.Membranipora monostacliys above and M. tenuis below). 



Enlarged twice. 



or piercing one of the valves. Rarely, in fossil 

 forms, the foot is absent and the animal is 

 fastened by the whole under shell. 



Mollusca : It is natural that in such a large 

 and variable group different methods of attach- 

 ment should have been evolved. Three methods 

 are common. First, bv means of hornv fibres 



AN ENCRUSTING BRYOZOAN 



(Lepralia paJlasiana) growing on a board. 



Enlarged twice. 



