1678 THE VOYAGE OF H.M.S. CHALLENGER. 



The family Castanellida represents a common and widely distributed group of 

 PH^ODARIA, which possess a very simple and uniform shell, viz., a simple lattice -sphere 

 with radial spines and one simple mouth. It may therefore be easily confounded with 

 the Coscinommida, or those Sphseroidea in which the shell is also a simple lattice- 

 sphere armed with radial spines (Astrosphcerida simplicia, p. 209). Indeed the shell 

 of both groups is very similar, and differs in one important point only ; the Castanellida 

 constantly possess one larger opening in the shell-wall, the shell-mouth, which is either 

 smooth or armed with a corona of teeth ; in the Coscinommida, however, such a mouth 

 is never present. The living specimens of both groups, and those shells in which the 

 soft body is preserved, are very easily distinguished, since the shell encloses in the 

 Coscinommida the central capsule and the transparent calymma only, whilst the latter, 

 besides, in the Castanellida contains a voluminous dark brown or green mass of 

 phseodella, the characteristic phseodium. A closer examination of the central capsule 

 reveals in all Castanellida the typical operculum, the astropyle, with the proboscis 

 of the PH.EODARIA, which is never present in any Sphseroidea. 



The Castanellida are easily distinguished also from those similar PH/EODARIA in 

 which the shell is also a lattice-sphere ; the lattice-work is constantly quite simple, as 

 in the similar Coscinommida, never composed of separated tangential pieces (as in the ' 

 A.ulospha3rida), or of porcellanous structure, with basal circles of pores (as in the 

 Circoporida), or of diatomaceous structure (as in the Challengerida). The gigantic 

 OrosphaBrida, which also in part possess a simple lattice -sphere, differ from the 

 Castanellida in the absence of the peculiar shell-mouth. 



Though the Castanellida belong to the most common PH^ODARIA, and though the 

 number of individuals, floating on the surface of the tropical seas, is extraordinarily 

 great, their variety of forms is very small; the six genera distinguished in the following 

 system differ only in very slight characters, and the majority of the species are very 

 similar, and often hardly distinguishable. The seven species figured in PL 113 

 exhibit the most striking differences which I could distinguish among all the species 

 observed. The shell usually has the characteristic appearance of a chestnut, a sphere 

 covered with very numerous short radial spines or bristles. In the majority of species 

 a certain number of longer thin radial spines is scattered over the surface ; these 

 are usually simple, rarely branched. The mouth of the shell, corresponding to the 

 proboscis of the central capsule, and placed in the same radius, is either a quite 

 simple larger opening with a smooth margin (figs. 3, 5, 7) or is armed with a corona of 

 teeth (figs. 1, 6). 



The size of the shell varies between - 2 and 0'8 mm., and is usually between 0'3 

 and - 5 mm. Its form is in the majority of species a geometrical sphere ; rarely it is 

 somewhat irregular, slightly ellipsoidal (prolonged in the axis of the mouth and 

 proboscis), or polyhedral (by conical protuberances from the bases of the radial spines). 



