262 



INVERTEBRATE MORPHOLOGY. 



- Tin 



av 



mouths are concerned, and it is possible to divide the Gymno- 

 Isemata into three groups or tribes, based upon these differ- 

 ences. In the tribe Gydostomata the zocecia are usually 

 cylindrical, and the mouth is circular and destitute of any 

 appendages ; iu the Otenostomata the mouth is closed during 

 retraction by a series of bristles which surround it {Alcyonid- 

 ium) ; while in the Cliilostomata, in which the ectocyst is 

 usually firm and frequently calcified, the mouth is closed by a 



lid, the operculum, furnished with spe- 

 cial muscles {Bugula, Menibranipora). 

 In this last-named tribe a poly- 

 morphism of the individuals compos- 

 ing a colony is frequently found. 

 Scattered among the ordinary indi- 

 viduals others, the Avicularia (Fig. 

 116, av), may be found having the 

 appearance of a bird's head, the 

 lower beak being fastened to the head 

 by a hinge and having inserted into 

 it strong muscles ; bunches of sensory 

 hairs are also present, and when these 

 are stimulated the lower beak is 

 rapidly snapped against the upper 

 and the stimulating organism thus 

 caught. There can be little doubt 

 but that these Avicularia are specially 

 modified individuals whose head and 

 upper beak represent the ordinary 

 individual, while the lower beak may 

 possibly be the equivalent of the 

 operculum ; physiologicallj- they have 

 been usually regarded as specialized 

 for the purpose of catching food for 

 the ordinary individuals, but it is not 

 improbable that their services may 

 rather be of a cleansing nature re- 

 moving from the colony particles of 

 dirt and the excreta, which by accumulating might interfere 

 with the proper function of the tentacles. Another polymor- 



Fm. 116.— PoKTioN 

 Colony op Bugula. 



OF A . 



a = anus. 

 av = avicularia. 

 bb = brown body. 

 ec = ectocyst. 

 / = funiculus. 

 oc = ovicell. 

 rm = retractor muscle. 

 / = tentacles. 



