94 MANUAL OF ZOOLOGY. 



gida in the sub-kingdom Protozoa, in which case they find their 

 most natural position in or close to the Rhizopoda. It is to be 

 remembered, however, that they differ from the other Protozoa 

 and agree with the Ccelenterata in the fact that the adult is 

 multicellular; that the ovum breaks up on fecundation into a 

 mass of primitive embryonic cells ; and that the larva passes 

 through a stage (" gastrula " stage) in which it is composed of 

 an outer and inner cellular layer, enclosing a central cavity, 

 which communicates with the outer world by a single opening. 

 It should be borne in mind, on the other hand, that there are 

 some undoubted Protozoa (e.g., some of the Radiolarians) which 

 have strong claims to be regarded as multicellular organisms. 



CHAPTER VI. 



INFUSORIA. 



THE Infusoria of many writers comprise many of the lowest 

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

 fer a > a class of minute animals now known to belong to the 

 Annulosa. 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 the fact that the members of the class are often developed 

 in organic infusions. 



The Infusoria or Stomatode Protozoa may be defined as Pro- 

 tozoa which are mostly provided with a mouth and rudimentary 

 digestive cavity, which do not possess the power of emitting pseudo- 

 podia, but which are furnished with vibratile cilia, or with con- 

 tractile filamejits.- They are mostly microscopic in size, the sarcode 

 is differentiated into an ectosarc and an endosarc, and a nucleus 

 and contractile vesicle are present. 



Most modern writers regard the Infusoria as strictly speaking 

 "unicellular" animals, each of the simple individuals corre- 

 sponding morphologically to a single cell. Upon this view 



