n 



PHYLUM PROTOZOA 



99 



and non-contractile (Fig. 72, 9, b), or may contain an axial 

 fibre or muscle, like that of Vorticella (Fig. 74, ax.f.). In Ophrydium 

 (Fig. 73, 4) the colony is an irregular mass, sometimes 4-5 in. in 

 diameter, consisting of a gelatinous substance in which a delicate, 

 branching stem is embedded, each branch terminating in a zooid. 

 Some genera (Fig. 73, 5) secrete a hollow, brown, gelatinous tube, 

 branched dichotomously ; the end of each branch is the habitation 

 of one of the zooids. 



Reproduction. Transverse fission is the universal method of 

 reproduction, the entire process taking from half an hour to two 

 hours in different species. In Vorticella (Fig. 74, E) and other 

 Peritricha the plane of division is parallel to the long axis of the 

 bell-shaped body. In such simple Peritricha as Vorticella division 

 proceeds until two zooids are produced on a single stalk ; one of 

 the two then 

 acquires a 

 second cir- 

 clet of cilia 

 near its prox- 

 imal end, be- 

 comes d e- 

 tached (E s ), 

 and, after 

 leading a 

 free - swim- 

 ming life for 

 a time, settles 

 down and 

 develops a 

 stalk : in this 

 way the dis- FIG. 

 persal of the 

 non -locomo- 

 tive species 

 is ensured. 



In many species of Zoothamnium (Fig. 75) the zooids are dimorphic : 

 the ordinary bell-shaped forms (n.z.) divide in the usual way, but as 

 they remain attached, the process results only in the increased com- 

 plexity of the colony, not in the development of a new one. The 

 larger zooids (r.z.) are globular and mouthless : they become 

 detached, swim off, and, after a short free existence, settle down, 

 develop a stalk (F), divide, and so form a new colony. 



Spore-formation takes place in Colpoda. The Infusor becomes 

 encysted, and divides into two, four, and finally eight masses, each 

 of which, becoming surrounded by a special investment, gives rise 

 to a spore. A somewhat similar process has been described in 

 Vorticella (Fig. 74, H) and others. 



H 2 



75. Zoothamnium arbuscula. 4,'entire colony ; B, the same, 

 natural size ; C, the same, retracted ; D, nutritive zooid ; E, repro- 

 ductive zooid ; F 1 , F%, development of reproductive zooid ; ax. f. 

 axial fibre ; c. vac. contractile vacuole ; nu. nucleus ; n.z. nutritive 

 zooid ; r.z. reproductive zooid. (From Parker's Biology, after Saville 

 Kent.) 



