COMPENDIUM. 289 



detection by the microscope : or that they may be dissipated after dis- 

 charging their contents : and that these contents will appear when in a 

 favourable medium. 



Yet all this resolving only into conjecture, let us stimulate the ener- 

 getic naturalist to farther researches, in expectation of finding the truth. 



The planula has been already described, as contained in a pod or 

 vesicle, borne principally by the hydraoid Sertularise. In some, the vesicles 

 amount to several hundreds on a specimen, and according to the species 

 of the product, each vesicle may contain from one to thirty planulae, 

 The planula is endowed with active motion when discharged from the 

 vesicle ; motion may continue several days, when it is arrested : its elon- 

 gated leech-like form shortens ; contracts to a spherical segment, from 

 the vertex of which a stalk speedily rises, crowned by a cell and hydra ; 

 and this is the originating figure of the hydraoid zoophytes. 



The process attending production of the planula by the Hydra squa- 

 mata, may be somewhat different ; also of an animal of corresponding for- 

 mation from another parent, the pectinate hydra of the Virgularia. But 

 this demands some farther investigation, for I have not yet succeeded in 

 ascertaining that other carnose zoophytes originate through the pla- 

 nular form. 



Many zoophytes are perpetuated by means of a small organic body 

 denominated the corpusculum or gemmule, for which no name more cha- 

 racteristic is yet adopted. 



By this is to be understood that certain beings are generated in the 

 body of the parent, endowed with the external properties of active life ; 

 spontaneous motion, apparent volition, and the like, on birth. 



But when viewing them at large, they are so different in form, and 

 belong to tribes so far apart, besides being subject to such diversified pro- 

 cesses in the course of conversion to perfect animals, that they cannot be 

 brought under general definitions. 



The gemmule is minute, consistent, soft, of variable shape, covered or 

 begirt by cilia, serving as natatory organs, and endowed with vigorous 

 locomotive faculties. Its activity is often very great, its movements rapid : 

 VOL. II. 2 o 



