XXX VI THE VOYAGE OF H.M.S. CHALLENGER. 



whilst, on the contrary, the polyzootic colonial Spumellaeia (or Polycyttaria) are 

 uninuclear only in the young state (PI. 3, fig. 12), and speedily present numerous sma 

 homogeneous nuclei, which have arisen by precocious division of a single nucleus ; these 

 are usually spherical and 0"008 to 0*012 mm. in diameter. The serotinous nucleus of the 

 monozootic Spumellaeia, in many divisions of this large legion, and especially in the 

 simply constituted Sphaeroidea, is a homogeneous sphere of niiclein, lying in the 

 middle of the central capsule. In many other cases it assumes the form of a spherical 

 vesicle (" Binnen-Blitschen "), whose fluid or semi-fluid contents are enclosed by a more or 

 less firm membrane. This vesicle often contains a single central spherical nucleolus 

 (PI. 1, figs. 1^, 4/), but sometimes a variable number of small excentric nucleoli (PI. 1, 

 figs, la, 2a). The nuclear membrane is often somewhat thick, presenting a double contour, 

 and in such cases may even exhibit a fine radial striation, the expression of minute pores 

 (PL 1, fig. 2a). In the colossal nuclei (as much as 1 to 2 mm. in diameter) of certain large 

 Thalassicollida the nucleolus presents a very remarkable form, becoming stellate by the 

 protrusion of processes, which may again branch in a dendritic fashion (as in the common 

 Thalassicolla nucleata), or it may develop into a very long cylindrical thread, which is 

 disposed in serpentine coils, and in Thalassophysa pelagica passes into the difi"erent 

 csecal processes of the stellate nucleus. In many Sphseroidea, whose skeleton is com- 

 posed of numerous concentric lattice sjDheres, the small central spherical nucleus lies at 

 first within the innermost of these (the medullary shell) ; but afterwards it grows through 

 the meshes of the lattice-work, and the radiating club-shaped processes thus formed 

 (PI. 11, fig. 5) unite with each other outside the medullary shell, and form an external 

 nuclear sphere which completely encloses the latter. In the Polysphserida (with several 

 concentric lattice-shells) and in the Spongosphserida (with spongy lattice-spheres), this 

 process may be several times repeated, so that eventually the central spherical nucleus 

 attains considerable dimensions, and encloses two or more concentric lattice-shells with 

 their radial connecting rods. The nuclear membrane is in these cases usually penetrated 

 by radial bars, which connect the outermost of the enclosed shells with the remaining 

 cortical shells which surround the central capsule. The same remarkable arrange- 

 ment is also very common among the Discoidea. The small spherical primary 

 nucleus is in such instances immediately surrounded by the innermost earliest developed 

 lattice-shell, around which the concentric rings are subsequently deposited; it then grows 

 out through the meshes, and the processes fuse outside the ring to form a homogeneous 

 lentiform nucleus (PL 43, fig. 15). The same process recurs in certain Prunoidea 

 and L arc o idea, whilst in other Spumellaeia of these groups {e.g., Pylonida) the 

 lobate processes of the nucleus remain free. 



Both the simple serotinous nucleus of the monozootic Spumellaeia, and the numerous pre- 

 cocious nuclei of the Polycyttaria, were first described in my Monograph in 1862, the former as 

 the " eudocyst " (" Binnen-Blaschen "), the latter as " spherical transparent vesicles " (" Kugelige 



