II 



PHYLUM PROTOZOA 



63 



Ctf .__: 



excreta. The central capsule encloses, in addition to the nucleus 

 or nuclei, oil-drops, vacuoles, proteid crystals, and pigment. 



In some genera the central capsule is the only skeletal structure 

 present, but in most cases there, is in addition a skeleton mainly 

 external formed, as a rule, of silica, but in one subdivision of the 

 class (the Acantharia) of a substance called acanlhin, composed of 

 strontium sulphate, so transparent that it can only b.e distinguished 

 from silica by chemical tests. The siliceous skeleton may consist 

 of loosely woven spines (Fig. 45), but" usually (and the acanthin 

 skeleton always) has the form of a firm frame-work of globular, 

 conical, stellate, or discoid shape, frequently 

 produced into simple or branched spines. 

 In the forms with an acanthin skeleton the 

 spines frequently have inserted into them 

 a number of contractile filaments arising 

 from the gelatinous extra-capsular layer. 

 A very beautiful form of skeleton is exhi- 

 bited by Actinomma (Fig. 47), in which 

 there are three concentric perforated spheres 

 (A, sk. 1, sk. 2, sk. 3) connected by radia- 

 ting spicules. The outer of these spheres 

 occurs in the extra-capsular protoplasm (B, 

 ex. caps, pr.), the middle one in the intra- 

 capsular protoplasm, and the inner one in 

 the nucleus (nu.). 



Colonial forms are comparatively rare 

 in this order, but occur in some genera by 

 the central capsule undergoing repeated 

 divisions while the extra-capsular mass 

 remains undivided. In this way is pro- 

 duced in Collozoum for instance (Fig. 48, 

 A, B, C) a firm gelatinous mass, the 

 calymma or vacuolated extra-capsular 

 protoplasm (D, vac.) common to the entire 

 colony, having embedded in it numerous 

 central capsules (c. caps.) each indicating a 

 zooid of the colony. Collozoum may attain 

 a length of 3 or 4 cm. 



Reproduction by binary fission has been observed in some 

 case^, and may be universal. The nucleus divides first, then 

 the central capsule, and finally the extra-capsular protoplasm. 



Spore-formation has been observed in Collozoum and some other 

 genera : the intra-capsular protoplasm divides into small masses, 

 each of which becomes a flagellula (Fig. 48, E, F) provided with 

 either one or two flagella. In some instances all the spores produced 

 are alike (E), and each encloses a small crystal (c.) : in other cases 

 (F) in the same species the spores are dimorphic, some being 



FIG. 45. Thalassoplancta 

 brevispicula, part of a 

 section, km. central cap- 

 sule ; ip. intra-capsular 

 protoplasm; n. nucleus, 

 containing nl. numerous 

 nucleoli ; of. oil drops ; ca . 

 calymma ; rp. protoplasm 

 surrounding calymma ; s. 

 spicules. (From Lang's 

 Comparative Anatomy , after 

 Haeckel.) 



