PROTOZOA : SPONGIDA. 69 



Mesozoic genera of sponges are Ventriculites and Siphonia; 

 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 Atnxbce. 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. This observer, in fact, states his 

 " conviction that the true ciliated Spongia are not Rhizopoda in 

 any sense whatever, nor even closely related to them, but are 

 genuine compound flagellate Protozoa." To prove this view, 

 however, it should be shown that each sponge-particle possesses 

 at any rate a distinct mouth, with or without a rudimentary 

 alimentary canal. More recently Dr Ernst Hasckel and others 

 have endeavoured to show that the sponges are most nearly 

 allied to the Sea-anemones (Actinozoa) ; 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 tlie Ccelen- 

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

 two are not of a very striking nature. In some cases (such as 

 Euplectella amongst 'Ca.tSilicispongice, sindSycum and (7te nmongst 



