ii PHYLUM PROTOZOA 103 



The only colonial form is the wonderful Dendrosoma (9), in 

 which the entire colony attains a length of about 2 mm., and bears 

 an extraordinary resemblance to a zoophyte (vide Section IV.). It 

 consists of a creeping stem from which vertical branches spring, 

 and the various ramifications of these are terminated in Podo- 

 phrya-like zooids with suctorial tentacles. The meganucleus is very 

 remarkable, extending as a branched axis throughout the colony 

 (6, nu.). Micronuclei of the ordinary character are present as well. 



Reproduction by binary fission takes place in many species. 

 In Ephelota gemmipara (8) a peculiar process of budding occurs : 

 the distal end of the organism grows out into a number of 

 projections or buds, into which branches of the nucleus extend. 

 These become detached, acquire cilia on one surface, and swim 

 off (6). After a short active existence tentacles appear and the 

 cilia are lost. In this case budding is external, but in Acineta 

 tuberosa (%, 2b) the buds become sunk in a depression, which is finally 

 converted into a closed brood-cavity (b.c.) : in this the buds take on 

 the form of ciliated embryos, which finally escape from the parent. 

 In Dendrosoma the common stem of the colony produces internal 

 buds (,9,6, bd.). 



Further Remarks on the Protozoa. 



The majority of the Protozoa are aquatic, the phylum being 

 equally well represented in fresh- and salt-water. They occur 

 practically at all heights and depths, from 8,000 to 10,000 feet 

 above sea-level to a depth of from 2,000 to 3,000 fathoms. Some 

 forms, such as species of Amoeba and Gromia, live in damp sand 

 and moss, and may therefore be almost considered as terrestrial 

 organisms. In accordance with their small size and the readiness 

 with which they are transported from place to place, a large pro- 

 portion of genera and even of species is 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 Ehizopoda, Mastigophora, and Infusoria 

 occur both as internal and external parasites. 



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 



