METAMORPHOSIS. 



27 



M 



tlie larva. Its covering of stiff setae probably serves some sensory 

 purpose, perhaps transmitting tactile impressions. 



b. Metamorphosis. 



The first indication that the larva is ready to attach itself is an 

 alteration in its mode of progression. The larva noAV constantly 

 moves in circles, with its retractile disc protruded to its utmost limit. 

 By a sudden contraction of the body the sucker is shot out, and the 

 fixation of the larva is accomplished by means of a sticky secretion 

 from its lower sur- 

 face. The evagi- 

 nated sucker (Figs. 

 10 and 11, s) in 

 Bugula shows at 

 first on its under 

 surface a circular 

 furrow, which sepa- 

 rates an outer 

 liroader part from 

 a narrower central 

 portion that pro- 

 jects downwards. 

 At a later stage, 

 the lower side of 

 the sucker flattens 

 into a broad cell- 

 plate {plaque ad- 

 hesive, Barrois), 

 from Avhich is de- 

 rived the basal sur- 

 face of the endocyst 

 of the primary 

 zooecium. 



The whole of the 

 rest of the body- 

 wall of the primary 

 zooecium, i.e., its 

 upper part and its 

 lateral walls, are 



yielded by the aboral portion which, in the larva, is comparatively 

 small. This takes place chiefly by an extension of those parts of 



Fig. 10. — Metamorphosis of the larva of Lepralia unicornis 

 (after J. Bakrois). A, first stage of metamorphosis. The 

 sucker (s) is evaginated, the corona (c) is beginning to bend 

 round. B, next (so-called umbrella) stage. The corona (c) is 

 comi>letely bent round, c, coronal cells ; o, ijyriform organ ; 

 p, matitle-cavity ; )•, retractile disc ; s, protruded sucker ; 

 V, vestibulum ; x, paired organ of doubtful significance 

 (according to Barrois, the mesodermal rudiment of the 

 polypide). 



