602 FOURTH REPORT — 1834. 



white and yellow respectively occur on different specimens of 

 the same species, and they are the most common of all. 



Preceding naturalists have maintained that the vesicles are 

 the ovaria, and the corpuscula the ova, whereby Sertularice are 

 propagated. But the author's experiments and observations 

 greatly multiplied during many years, on many specimens of 

 various species, have not led the author directly to the same 

 conclusion. 



The contents of the vesicle are not distinguished originally 

 by any definite form. At a certain stage they are recognised 

 as spherical or ovoidal corpuscula, the former being their 

 earliest sensible shape. In advancing somewhat further, they 

 resolve into spherical triangular prisms, betraying evident ani- 

 mation by extension and contraction ; and motion commencing, 

 places are interchanged among them. At length, attaining 

 maturity, they issue from the orifice of the vesicle, not as 

 young iSertidaricB, but as a race of perfect animals, bearing 

 many features of the Planaria, and which may constitute a new 

 genus, to be denominated Planula. 



The vesicle now remains empty and transparent, nor is it 

 known to be replenished by another brood ; but occasionally a 

 small portion appears in the centre, as of a slender twig having 

 penetrated upwards. 



These creatures are produced in extraordinary profusion. 

 Above 1200 have issued from the vesicles of a portion of the 

 Sertularia Uber; and multitudes, in still greater proportion, 

 from those of others. But only one was contained in the 

 vesicles of the Sertularia abietina examined, and in those of 

 several species which did not attain maturity. 



Ordinarily the Planulce are white or yellow, opake, smooth, 

 and flattened, somewhat triangular, tapering from the head 

 which is always obtuse, downwards to the lower extremity, 

 and extending from inferior dimensions to rather above a line. 

 Those of certain species are pyriform, and of others linear, 

 with obtuse extremities. Neither eyes nor any external or- 

 gans have been discovered. 



The motion of the Planulce is smooth and gliding; they crawl 

 actively over the bottom of the containing vessel or up its 

 sides ; they suspend themselves in the water by an invisible 

 thread, as the Planarice, and like them swim supine. 



But in a few days their motion relaxes ; they become sluggish 

 and stationary ; their figure alters, and they die, yet without 

 that speedy decomposition incident to Planarice. 



Very soon afterwards a circular spot or low spherical segment 

 of the same colour, white or yellow, is observed in just about 



