RADIOLARIA. 



21 



the Radiolarian. They have been named Zooxantliella nutricola, and 

 contain chlorophyl, a nucleus and a cellulose wall. The central capsule, 

 which may be either conical or spherical, is either perforated by fine 

 pores over its whole circumference (Peripylaria), or the ppes are 

 limited to a definite part of its surface (Monopylaria\ or there are 

 only a few, usually three, large pores (Tripylaria). 



Many Radiolaria form colonies, and are composed of numerous 

 individuals. In such colonies the extracapsular protoplasm is united 

 with that of neighbouring individuals, so that the Avhole colony may 

 be described as consisting of a common mass of vacuolated protoplasm 

 containing in itself, not as in the moiiozoic Radiolaria a single central 

 capsule, but a number of capsules. 



The whole animal in the solitary as well as in the colonial forms is 

 embedded in a structureless jelly, called the Calymma. The extracapsular proto- 

 plasm may be described as consisting of the following parts : (1) the Sarcomatrix, 

 the layer which surrounds the central capsule ; (2) the Sarcodictyum, the layer 

 which bounds the outer surface of the calymma ; (3) the Sarcoplegma, or anasto- 

 mosing threads which traverse the calymma and connect the sarcodictyum and 

 sarcomatrix. From the extracalymmar sarcodictyum proceed the pseudopodia. 



Only a few species remain naked and without firm deposits; as 

 a rule, the soft body possesses a skeleton, which is composed either 

 of silica or of an organic 

 substance called Acanthin, 

 and either lies entirely 

 outside the central capsule 

 (Ectolithia) or is partially 

 within it (Entolithici). In 

 the most simple cases 

 the skeleton consists of 

 small, simple, or toothed 

 silicious needles (spicula) 

 united together, which 

 sometimes give rise to a 

 fine sponge -work round 

 the periphery of the proto- 

 plasm, e.g., Physematium. 

 In a higher grade we find 

 stronger hollow silicious 



spicules, which radiate from the middle point of the body to the 

 periphery in regular number and order, e.g., AcantJwmetra. (Fig. 17.) 

 A fine peripheral framework of spicules may be added to these. 

 In other cases simple or compound lattice-works, and perforated 



FIG. If.Acanthoimtra Miilleri (after E. Haeckel). 



