PHYLUM PROTOZOA 



61 



the perforated calcareous shell of Hastigerina with its investment 

 of vacuolated protoplasm. It is found in its simplest form in 

 Thalassoplancta (Fig. 44), in which it is spherical and uniformly 

 perforated with minute holes. In other forms, such as Lithocircus 

 (Fig. 43), it is 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. 45), 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 operculum (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 pigmented 

 matter called the phmodium (ph.), prob- 

 ably partly of the nature of 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 

 of a substance called acanthin, composed 

 of strontium sulphate, so transparent that 

 it can only be distinguished from silica 

 by chemical tests. The siliceous skeleton 

 may consist of loosely woven spines 

 (Fig. 44), 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 fre- 

 quently have inserted into them a number 

 of contractile filaments arising from the 

 gelatinous extra-capsular layer. A very 

 beautiful form of skeleton is exhibited by 

 Actinomma (Fig. 46), in which there are 

 forated spheres (A, sk. 1, sk. 





Fig. 44.— Thalassoplancta 

 brevlspicula, part of a 

 section. Icin. central cap- 

 sule ; ij>. Intra-capsular 

 protoplasm ; n. nucleus, 

 containing nl. numeroiis 

 nucleoli ; lit. oil drops ; ca. 

 calymma ; rp. protoplasm 

 surrounding calymma ; s. 

 spicules. (tVom Lang's 

 Comparat ive A natomy, after 

 Haeckel). 



three concentric per- 

 , sfc .3) connected by radiating spicules. 

 The outer of theW 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 produced— in CoUozoum for instance (Fig. 47, A, B, C) 



