76 MANUAL OF ZOOLOGY. 



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 " indi- 

 viduality" 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 reasonable view. 

 Thirdly, it may be held that each sponge mass consists of a 

 number of aggregated " individuals," each of which is con- 

 stituted 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 Ccslen- 

 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 the Silicispongice, and Sycum and Ute 

 amongst the Calcispongue) the adult sponge never consists of 

 more than a single osculum and the pores belonging to it, con- 

 stituting what Haeckel terms a single " person." Most sponges, 

 however, form a " stock," consisting of several such " persons " 

 united together. 



CHAPTER VI. 



INFUSORIA. 



THE Infusoria of many writers comprise many of the lowest 

 forms of plants such as the Diatoms together with the Roti- 

 fera, a class of minute animals now known to belong to the 

 Annuloida. By modern writers, however, the term Infusoria 

 is used strictly to designate those Protozoa which possess a 

 mouth and rudimentary digestive cavity. They are, for this 

 reason, often called collectively the " stomatode " Protozoa, in 

 contradistinction to the remaining members of the sub-kingdom, 

 which are all " astomatous." The so-called " suctorial " Infu- 

 soria (Acinetce), however, appear to have no definite oral aper- 

 ture ; and the same is the case with the parasitic Opalina, 

 though there is great doubt as to the propriety of placing this in 

 the Infusoria at all. The name Infusoria itself is derived from 



