PROTOZOA : SPONGIDA. 65 



monest genus. (The Devonian Steganodutyum is really the 

 cephalic buckler of a pteraspidean fish.) The most important 

 Mesozoic genera of sponges are Ventriculites and Siphania ; 

 and the order appears, upon the whole, to attain its maximum 

 in the Cretaceous epoch. There seems no reason to doubt 

 but that many of the chalk-flints owe their origin to sponges ; 

 and in some sections of flint are found minute " spherical bodies 

 covered with radiating and multicuspid spines," which have 

 been termed Spiniferites or Xanthidia, and are probably the 

 " gemmules " of sponges. (By some, however, these bodies are 

 regarded as being the "sporangia" of Desmidice, an order of 

 the Protophyta.) Many Cretaceous and Tertiary shells are 

 found to be mined by a species of boring sponge, which is 

 nearly allied to the recent Cliona. 



AFFINITIES OF SPONGES. As already pointed out, the 

 sponges are allied both to the Amcebea and to the Foraminifera. 

 Indeed the individual "sarcoids"or sponge-particles can scarcely 

 be distinguished, when detached, from Amoebce. The sponges 

 show likewise a decided relationship to the Radiolaria ; and 

 by Professor James-Clark they are believed to be nearly allied 

 to the " flagellate " Infusoria. More recently Dr Ernst Haeckel 

 and others have endeavoured to show that the sponges are 

 most nearly allied to the Sea-anemones (Actinozod) ; but this 

 seems to have arisen from a misconception as to the compound 

 nature of the former. Three views, namely, may be held as to 

 the " individuality " of a sponge. Firstly, it may be held that 

 the entire organism which we call a sponge, is a single animal. 

 The microscope has rendered this view wholly untenable. 

 Secondly, it may be held that the entire sponge-mass is a single 

 " zoological individual," of which each sarcoid is a single 

 " zooid." As each sponge-mass is certainly in most cases the 

 product of a single ovum, this is the most probable and reason- 

 able view. Thirdly, it may be held that each sponge-mass 

 consists of a number of aggregated " individuals," each of which 

 is constituted by a single exhalant " osculum," together with 

 the greater or less number of inhalant " pores " thereto apper- 

 taining. Upon no other view than this does there appear to 

 be any relation of affinity between the sponges and the Ccelen- 

 terata ; and even on this view the general affinities between the 

 two are not of a very striking nature. 



VOL. I. 



