BRYOZOA. 99 



The function of respiration must be referred to that part of the 

 body which is provided with the means of effecting a constant re- 

 newal of the surrounding oxygenized medium upon its surface. In 

 the ciliated tentacula, whose currents. Dr. Farre observes, seem much 

 beyond what is necessary to afford a sufficient supply of food, we, 

 therefore, recognise the principal respiratory as well as prehensile or- 

 gans. The currents of the nutrient fluids which may traverse their 

 interior canal would thus be more effectually exposed to the influence 

 of the surrounding medium. 



The individuals of the Bryozoa are multiplied by two processes of 

 generation ; the one by gemmae or buds from the common stem, which 

 appears to be uninfluenced by season, and which increases the size of 

 the aggregate mass of the Bryozoon ; the other by the liberation of 

 the young in the form of locomotive ciliated gemmules, which takes 

 place at certain seasons, generally in spring. 



In the Flustra the gemmee are developed from the cells of the 

 pre-formed individuals ; but in those Bryozoa which have connecting 

 stems the buds arise from the stem. They are at first homogeneous ; 

 then a distinction may be observed between the cell (^Jig- 51. C, a) 

 and the visceral contents (b) ; afterwards the tentacles may be dis- 

 cerned, which are at first short and stumpy ; finally, the cavity, walls, 

 and divisions of the alimentary canal become distinguishable. 



In regard to the generation by locomotive gemmules, these are 

 doubtless originally developed from fertile ova. 



Certain phenomena have been observed in the Bryozoa which 

 justify the belief that the individual polypes are male and female. 

 Dr. Farre has figured a specimen of the Valkeria cuscuta, in which he 

 observed a very remarkable agitation of particles in the visceral 

 cavity, caused by a multitude of minute cercarias swimming about 

 with the greatest activity in the fluid with which that cavity is filled : 

 they consisted simply of a long slender filament with a rounded ex- 

 tremity, by which they occasionally fixed themselves. Similar moving 

 filaments were not un frequently observed in other species. On one 

 occasion Dr. Farre observed them in a specimen of HalodactyluSy 

 drifting rapidly to the upper part of the visceral cavity, and issuing 

 from the centre of the tentacula, indicating an external communication 

 with the cavity of the body. The analogy of these cercariee with the 

 spermatozoa discovered by Wagner in the tortuous generative tubes 

 of the Actinia, indicates their real nature and importance in the 

 generative economy of the Bryozoa. 



The development and vital phenomena of the reproductive gem- 

 mules have been studied with most completeness in the Halodactylus. 

 They appear in spring as minute whitish points just below the surface, 



H 2 



