PHYLUM PROTOZOA 



59 



more or less conical in form, and the apertures are restricted to the 



flat base of the cone. Lastly, in the most complex forms (Fig. 42), 



the membrane of the capsule is double, 



and there are three apertures a principal 



one having a central position and provided 



with a lid or opcrculum (op.), and two 



subsidiary ones on the opposite side. In 



relation with the principal or lidded 



aperture there is found in the extra- 



capsular protoplasm a heap of pigment 



called the phwodiwn (pli.\ 



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 

 of a chitinoid substance called acanthin, so 

 transparent that it can only be distin- 

 guished from silica by chemical tests. The 

 siliceous skeleton may consist of loosely 

 woven spines (Fig. 41), but usually (and 

 the acanthin skeleton always) has the 

 form of a firm frame-work of globular, 

 conical, stellate, or discoid shape, fre- 

 quently produced into simple or branched 

 spines. A very beautiful form of skeleton 

 is exhibited by Actinomma (Fig. 43), in 

 which there are three concentric per- 

 forated spheres (A, sL 1, sk 2, sk. 3} con- 

 nected by radiating 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 divi- 

 sions while the extra-capsular mass remains undivided. In this 

 way is produced in Collozoum for instance (Fig. 44, A, B, C) a 

 firm gelatinous mass, the calymna 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 T>y binary fission has been observed in some cases, 

 and is probably 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, 



FIG. 41. Thalassoplancta 

 brevispicula, part of a 

 section, km. central cap- 

 sule ; if), intra-capsular 

 protoplasm ; n. nucleus, 

 containing nl. numerous 

 nucleoli ; ot. oil drops ; ca. 

 calymna ; rp. protoplasm 

 surrounding calymna ; s. 

 spicules. (From Lang's 

 Comparative Anatomy , after 

 Haeckel). 



