liv THE VOYAGE OF H.M.S. CHALLENGER. 



The clear vesicles to which this is due are either spherical, or polyhedral from mutual 

 pressure, and like the similar ones in the central capsule may be divided into membrane- 

 less vacuoles and vesicular alveoles. The vacuoles are simple drops of fluid, without a 

 special envelope, and immediately surrounded by the gelatinous substance of the 

 calymma, in which they appear as simple cavities. The alveoles on the contrary are 

 true vesicles, with a thin envelope, which encloses a drop of fluid or a globule of jelly; 

 in the latter case its contents are different in refracting power and amount of contained 

 water from the substance of the surrounding calymma. A sharp boundary between the 

 membraneless vacuoles and the vesicular alveoles cannot be drawn in the case of the 

 extracapsular hyaline spheres any more than in the intracapsular ; the envelope of the 

 alveoles is sometimes very distinct and even anatomically separable, whilst at other 

 times it is very thin and scarcely recognisable ; it may occasionally arise and disappear 

 within a very short time (see note A). There is no doubt that in the calymma as in 

 the central capsule the vesicular alveoles are secondary products, which have arisen 

 from the vacuoles by the secretion of an enveloping membrane. This membrane is 

 either a delicate sheath of exoplasm, or a firmer and more resistant skin, distinct from 

 the exoplasm, and probably an excretion from it (e.g., PL 4, figs. 2, 3). In many 

 cases the outer surface even of the vacuoles is covered by a network of pseudopodia, 

 which form a sarcoplegma similar to a fenestrated alveolar membrane. The colourless 

 pellucid fluid in the vacuoles and alveoles is usually simple sea-water, more rarely it 

 contains a small quantity of albumen (" albumen -spheres ") or jelly (" gelatinous 

 spheres "). The size of these spheres is very variable. Quite small vacuoles may be 

 found in the calymma of many Radiolaria. Large vacuoles, on the other hand, 

 producing the appearance of an alveolar structure, are confined to but few groups, to a 

 part of the Spumellaeia (Colloidea, Beloidea, and a few Sphairoidea), and 

 to the Phseocystina (Ph^odaria with incomplete skeleton) ; besides they occur 

 only rarely in individual genera, e.g., Nassella among the skeletonless Nassellaria. 

 Since the volume of the calymma is much increased by the development of vacuoles, 

 and the power of mechanical resistance is at the same time much increased, the fact is 

 explained that the vacuoles occur mainly in Radiolaria which have no skeleton or only 

 an incomplete one (see note B). Among the monozootic Collodaria the alveolar struc- 

 ture is especially well developed in the following genera ; Tlialassicolla (PI. 1, figs. 4, 5), 

 Tlialassophysa, Thalassoplancta, Lampoxanthium (PL 2, figs. 1, 2); among the 

 Ph^eodaria in most genera of the Phseodinida, Cannorrhaphida and Aulacanthida 

 (Pis. 101-104), and probably also in other voluminous Ph^eodaria (e.g., P h jse o- 

 s p h se r i a). The alveoles or vacuoles in the calymma of these large Radiolaria lie 

 usually in several layers, one above another, and increase in size from within outwards. 

 The Polycyttaria or social Radiolaria (the three families CoUozoida, Sphajrozoida and 

 CoUosphjerida) without exception have an alveolar structure, and the special form of 



