62 



ZOOLOGY 



ORDER 5. RADIOLARJA. 



SECT. 



The Radiolaria are a large and well-defined group of Rhizopods, 

 noticeable, in most instances, by the presence of a siliceous skeleton 

 of great beauty and complexity. They are all marine, 



General Structure. The most important characteristic of 

 the group is the presence of a perforated membranous sac, called 

 the central capsule (Fig. 44, cent, caps.), which lies embedded in the 

 protoplasm, dividing it into intra-capsular (int. caps, pr.) and extra- 

 capsular (ext. caps, pr.) regions. In the intra-capsular protoplasm 

 is a large and complex nucleus (nu.), or sometimes many nuclei : 

 from the extra-capsular protoplasm the pseudopods (psd.) are given 

 off in the form of delicate radiating threads, which in some cases 

 remain free, in others, e.g. Lithocircus, anastomose freely, i.e. unite 

 to form networks. In one large section the Acanlharia the 

 pseudopodia, or some of them, contain firm axial rods similar to 

 those in the pseudopods of the Heliozoa. There is no contractile 

 vacuole, but in many forms the extra-capsular protoplasm contains 



numerous large non-con- 

 tractile vacuoles, which give 

 it the frothy or bubbly ap- 

 pearance noticed previously 

 in Hastigerina. The vacuo- 

 lated portion of the proto- 

 plasm has a gelatinous 

 consistency, and is distin- 

 guished as the calymma. In 

 the Acanthometridee a 

 gelatinous sheath formed of 

 an extension of the calymma 

 invests the spicules of- the 

 skeleton. 



The central capsule may 

 be looked upon as a chiti- 



noid internal skeleton, reminding us of the shell of Gromia and of 

 the perforated calcareous shell of Hastigerina with its investment 

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

 Thalassoplancta (Fig. 45), in which it is spherical and uniformly 

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

 (Fig. 44), 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. 46), 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 pigmeuted 

 matter called the phceodium (ph.), probably partly of the nature of 



Steel.' 



m 



^fixt.caps.pr. 



M(i. 44. Lithocircus annular is. cent. caps. 

 central capsule ; ext. caps. pr. extra-capsular 

 protoplasm ; int.. caps. pr. intra-capsular pro- 

 toplasm ; nu. nucleus ; psd. pseudopods ; 

 skel. skeleton ; z. cells of Zooxanthella. (After 

 Biitschli, from Parker's Biology.) 



