1680 THE VOYAGE OF H.M.S. CHALLENGER. 



thickness is in the majority of species al)out equal to the diameter of one or two pores 

 of the hittice. Sometimes they are much stronger, rarely thinner. 



The form of the radial main-spines is not very variable, usually it is cylindrical or 

 cylindro-conif^al, rarely angular, prismatic or pyramidal. Sometimes they are straight 

 (perfectly radial), at other times more or less irregularly curved. Usually they are 

 simple, rarely branched ; the branches are always very irregular and exhiljit a remark- 

 able tendency to unite by concrescence (figs. 3, 4). Some thicker forms of spines 

 exhibit a remarkable structure, the surface being covered with small dimples and 

 sj)inules between them (fig. 1); sometimes the dimples are hexagonal (fig. 5a). In 

 the axis of the thicker spines there is often \'isible the same axial filament or funirle 

 which we find in many other Ph.eodaria, more rarely a hollow axial canal. 



The mouth of the shell is usually circular, more rarely polygonal or irregularly 

 roundish (fig. 2). It is either quite smooth (figs. 3, 5, 7) or armed with a corona of 

 strong teeth (figs. 1, 4, 6). These teeth are diflerent from the other spines of the 

 shell ; they are much thicker than the radial bristles, and shorter than the main-spines. 

 Their numljer varies between three and twenty, usually between five and seven. They 

 are usually parallel to the main axis, which is determined by the excentric position of 

 the mouth and the centre of the sphere (figs. 1, 6). The teeth are usually conical, 

 more or less irregular in size and form, more rarely of equal size. The corona of teeth 

 is usually sharply separated from the surrounding lattice-work of the shell, but has no 

 influence on its regular spherical form. 



The central capsule of the Castanellida immediately determines bej^ond doubt their 

 true Ph.eodarian nature ; it lies excentrically in the cavity of the enclosing shell,, 

 and is .surrounded at the oral pole by the large aiid dark phajodium. The latter covers 

 the radiate operculum of the capsule membrane and the tubular proboscis, which arises 

 from it and is directed towards the shell mouth. Usually the diameter of the spherical 

 central capsule is about one-third of the shell diameter, rarely one-quarter only, and in a 

 few cases it attains nearly its half. The calymma, therefore, is larger than the capsule, 

 and fills up the whole cavity of the shell which is not occupied by the latter. The 

 greater part of the calymma again is filled up by the dark green or brown, sometimes 

 black phseodella, the roundish granules which compose the voluminous nou-transjDarent 

 phseodium. Sometimes the latter proceeds through the mouth and is partially placed 

 outside the shell (fig. 5). 



The astropyle, or the main-opening of the central capsule, is placed on the oral j^ole of 

 its main axis, while its radiate operculum and the tubular proboscis arising from it possess 

 the same structure as in the other Ph.eodaeia. But the two lateral parapylse or accessory 

 openings which are present in the majority of the latter seem to be absent in the 

 Castanellida as well as in the Medusettida and the Challengerida. I could never observe 

 a trace of them. The proboscis is a small cylindrical tube and is directed towards the 



