PHYLUM PROTOZOA 



97 



attached to the interior of the tube, and is closed by a contractile 

 thread of protoplasm (w.), which acts as a retractor muscle. 



Compound forms or colonies are common among the Peritricha, 

 rare in the other subdivisions. Many peritrichous forms occur as 

 branched, tree-like colonies, often of great complexity (Fig. 71, Pa; 

 Fig. 74). The stem of these may be a purely cuticular structure 

 and non-contractile (Fig. 71, 9, J), or may contain an axial 

 fibre or muscle, like that of Vorticella (Fig. 73, ax.f.). In Ophridium 

 (Fig, 72, 4) tne colony is an irregular mass, sometimes 3-4 cm. in 

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

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

 Some genera (Fig. 72, 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 



FIG. 74. _ Zoothamnium arbuscula. A, entire colony; B, the same, natural size; C, the 

 same, retracted ; D, nutritive zooid ; E, reproductive zooid ; F 1 , F2, development of reproduc- 

 tive zooid; ax.f. axial fibre ; c. roc. contractile vacuole ; nu. nucleus; n,z. nutritive zooid; 

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



hours in different species. In Vorticella (Fig. 73, E) and other 

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

 bell-shaped body, but as the distal surface probably corresponds 

 with the dorsal surface of such forms as Paramoecium, fission 

 is really transverse in this case also. 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 circlet 

 of cilia near its proximal end, becomes detached (E 3 ), and, after 

 leading a free-swimming life for a time, settles down and develops 

 a stalk : in this way the dispersal of the non-locomotive species is 

 ensured. In many species of Zoothamnium (Fig. 74) the zooids 

 VOL. I H 



