Mr. W. S. Kent 07i the Embryology of Sponges. 143 



Monosiga angustata^ S. K.*, with an isolated unit or zooid from 

 the sponge-gemmule in the same siraplj flagellate and collar- 

 bearing states. But for the accompanying explanation, in- 

 deed, the two might be interpreted as representing slightly 

 varying individuals of the same specific type. Borrowing a 

 simile from the vegetable kingdom, this matured and liberated 

 sponge-gemmule presents now, as it swims through the water, 

 a structural composition broadly corresponding with that of 

 Volvox glohator. The organism, as a whole, is propelled by 

 the vibratory movements of the associated flagella ; while, in 

 the same manner, each unit of the compound body, viewed 

 separately, exhibits that relationship towards Monosiga and 

 other independent collar-bearing monads which is borne by 

 those of Volvox with reference to such solitary types as Di- 

 sehnis or Chonemonas. Sooner or later, the sponge-gemmule 

 having transported itself, by aid of the concerted action of the 

 countless vibratile flagella, to a spot suitable for attachment, 

 the collars and flagella of the separate monads are retracted, 

 and the organism becomes fixed, usually by one extremity, to 

 the chosen fulcrum of support. An exuded veil of sarcode or 

 syncytium is now poured out, hiding the monads from super- 

 ficial view, and the transformation of the gemmule into a 

 typical sponge-stock, as already detailed by Mr. Carter [l. c. 

 p. 334 et seq.), is speedily effected. 



In no one of the several phases passed through by this so- 

 called sponge-embryo, as here recounted, can there be said to 

 have been the formation of any distinct membrane produced by 

 the uniting into one morphological whole of the cellular units 

 or segmentation-masses, such as takes place invariably among 

 all Metazoa, each of the separate units of this segmented body 

 exhibiting a totally separate and independent existence. The 

 only presumed metazoic characteristic manifested, indeed, by 

 this ciliated structure is its primary assumption by continual 

 subdivision of a morula-like condition. This moruloid condi- 

 tiouj however, can be shown to be common to many undoubted 

 Protozoa as well as Metazoa, the distinction between the two 

 groups depending therefore upon the circumstance whether 

 or not the component segments or blastomeres of this morula- 

 like body maintain a separate existence, or become welded 

 into a single continuous tissue or blastoderm. Among those 

 conspicuous instances in wdiich a moruloid condition is ex- 

 hibited by undoubted Protozoa, attention may be first directed 



* An illustration of this and nearly forty other independent collar- 

 bearing monads will be found accompanying an article on these newly 

 discovered organisms, contributed by the writer to the ' Popular Science 

 Review ' for April 1878. 



