Page:Scientific results HMS Challenger vol 18 part 1.djvu/300

92

Definition.— living associated in colonies, united by an alveolar jelly-body, and connected by the network of anastomosing pseudopodia.

The family comprises all polyzous or social, and constitutes the only polyzoic group among the. This group was first constituted by J. Müller as "Radiolaria polyzoa with shells." Formerly following his authority, in my Monograph I had separated them from the other and united them with the social Collodaria (Sphærozoida). Also R. Hertwig in his Organismus der Radiolarien united them with his Sphærozoea. In my Prodromus I had retained this isolated position. But a further careful study has convinced me that this isolation is not truly natural, and that the Collosphærida are only "social Ethmosphærida" which have arisen from this solitary subfamily by adaptation to colonial life. There are some forms of Collosphærida which are nearly identical with some forms of Ethmosphærida, only differing from the latter by their association in colonies; and in some forms of both groups it is quite impossible to decide whether the isolated shells appertain to one or to the other family.

The isolated shell of the Collosphærida is almost constantly (with few exceptions) a simple extracapsular lattice-shell, as in the Monosphærida; only the small group of Clathrosphærida (with the genera Clathrosphæra and Xanthiosphæra) exhibit an exception, the simple lattice-shell being overgrown by an external mantle or veil of very thin, cobweb-like, irregular lattice-work (Pl. 8, figs. 6-11). Therefore these Clathrosphærida bear to the Acrosphærida (or the common simple Collosphærida) a relation similar to that which Liosphæra (p. 76) bears to Cenosphæra; both shells are extracapsular "cortical shells" at a very short distance apart. In the Collosphærida true concentric medullary shells never occur; the central capsule always lies quite freely in the simple or double cortical shell, separated from it by a jelly-veil.

Although a well marked difference in the simple lattice-shell of the social Collosphærida and the solitary Ethmosphærida does not exist, nevertheless in most cases the two shells can be distinguished by a practiced observer. The simple fenestrated shells of the monozoic Ethmosphærida are commonly quite regular spheres in a mathematical sense, or regular "endospherical polyhedra"; whereas in the Collosphærida they are commonly more or less irregular, often to an extraordinary degree (Pls. 5-8). Some species of Collosphærida, however, also possess quite regular spherical shells. Another difference is often shown in the lattice-work of the shells, which in the Collosphærida is nearly always very irregular, and exhibits a peculiar tendency to the