102 ZOOLOGY sect. 



portion of genera and even of species are universally distributed, 

 being found in all parts of the world where the microscopic fauna 

 has been investigated. 



Numerous parasitic forms are known. Besides the entire class 

 of Sporozoa, species of Rhizopoda, Mastigophora, and of Infusoria 

 occur both as internal and external parasites. Species of Amoeba 

 are common in the intestines of the higher animals, and one 

 species has been found in connection with a cancerous disease in 

 Sheep. A ciliate Infusor, Ichthyophthirius, is found in the skin of 

 freshwater Fishes, where it gives rise to inflammation and death. 



Many instances have been met with in our survey of the 

 Phylum of compound or colonial forms, the existence of which 

 seems at first sight to upset our definition of the Protozoa as 

 unicellular animals. But in all such cases the zooids or unicellular 

 individuals of the colony exhibit a quasi-independence, each, as a 

 rule, feeding, multiplying, and performing all other essential 

 animal functions independently of the rest, so that the only 

 division of labour is in such forms as Zoothamnium and Volvox, 

 in which certain zooids are incapable of feeding, and are set apart 

 for reproduction. In all animals above Protozoa, on the other 

 hand, the body is formed of an aggregate of cells, some of which 

 perform one function, some another, and none of which exhibit 

 the independent life of the zooid of a protozoan colony. It cannot, 

 however, be said that there is any absolute distinction between a 

 colony of unicellular zooids and a single multicellular individual : 

 Proterospongia and Volvox approach very near to the border-land 

 from the protozoan side, and a similar approach in the other 

 direction is made by certain animals known as Mesozod, which will 

 be discussed hereafter (Sect. IV.). Moreover, the Mycetozoa, the 

 plasmodia of which are formed by the fusion of Amcebulae, the 

 nuclei of the latter remaining distinct and multiplying, are rather 

 non-cellular than unicellular. This point will also be referred to 

 at the conclusion of the section on Sponges (Sect. III.). 



In each division of the Protozoa we have found comparatively 

 low or generalised forms side by side with comparatively high or 

 specialised genera. For instance, among the Rhizopoda, there 

 can be no hesitation in placing the Lobosa, and especially Prota- 

 mceba, at the bottom of the list, and the Radiolaria at the top. 

 Similarly, among the Mastigophora, such simple Flagellata as 

 Oikomonas (Fig. 52. # and 8) and Heteromita are obviously the 

 lowest forms, Noctiluca and the Dinoflagellata the highest. But 

 whether the Rhizopoda, as a whole, are higher or lower than the 

 Flagellata, is a question by no means easy to answer. A flagellum 

 certainly seems to be a more specialised cell-organ than a 

 pseudopod, and some of the Mastigophora rise above the highest 

 of the Rhizopoda in the possession of a firm cortex and cuticle, 



