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



most is a medullary shell. In this case the size of the capsule remains intermediate 

 between the inner and the middle shell. 



The T e t r a s p h te r i d a, or the S p h se r o i d e a with four concentric shells, are in 

 general not frequent, and not rich in different forms. In most of the observed species 

 two inner shells are medullary, two outer cortical shells, the former within, the latter 

 without, the central capsule ; and the wall of the capsule, pierced by the connecting radial 

 beams, lies between the two middle shells. But there are some Tetrasphserida in which 

 all four shells seem to be external or cortical shells. 



The P o 1 y s p h 06 r i d a, or the Sphseroidea with five or more concentric shells, 

 seem of course to offer the greatest possibility for the development of very different 

 forms; but in reality this group is the poorest and smallest of all; and only one part of it, 

 the Arachnosphserida, is rather common. In this peculiar division the shell is composed 

 of five to ten or more, very delicate, cobweb-like concentric shells, which are connected 

 by radial beams ; all are cortical shells, and lie outside the central capsule. Much 

 more rare are those Polysphajrida, in which both innermost shells, as true medullary 

 shells, lie within the central capsule, all others being outside it. The total number of 

 concentric shells in this group is commonly between five and ten, rarely more. 



The Spongosphserida are distinguished from all other S p h se r o i d e a by the 

 spongy structure of the spherical shell, which is composed wholly or partially of an u-regular 

 spongy framework. The relation of this group to the other groups of S p h se r o i d e a 

 is probably rather complicated, for in some Spongosphaerida the whole shell is composed of 

 massive spongy reticulation, whilst in others it contains a spherical central cavity, and in 

 a third group this cavity is filled up by one or two concentric lattice-shells, connected by 

 radial beams. Many of these Spongosphserida are very common, and of considerable size. 



The Collospheerida form a peculiar separate group of S p h ae r o i d e a, distin- 

 guished from all others by their social life or aggregation in colonies (coenobia). The}' 

 represent the only group of Sphserellariain which this association of numerous 

 individual capsules or cells is realised. The shell is almost constantly simple, without 

 regularly disposed radial spines; therefore they may be called "social Monosphserida," or 

 better "polyzoic Ethmosphserida." Only in one small group (Clathrosphserida) the shell, 

 enveloping every central capsule, is double or surrounded by an external mantle ; these 

 may be compared to the Dijalosphserida (or better to a part of the Carposphserida, 

 LioRphcera, p. 76). In most of the CoUosjihserida the lattice-shell is more or less irregular 

 in form and structure. 



Tlie Lattice Work of the fenestrated shells is in the S p h as r o i d e a of the greatest 

 variability, and its innumerable modifications serve mainly for the distinction of species. 

 In general we can distinguish as the most important modifications a regular network 

 (with equal size, form, and distance of the pores or meshes) and an irregular network (with 



