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



spbaerida (PI. 2), and the colonial CoUosphaerida with a Spheeroid skeleton (Pis. 5-8) 

 from the solitary Ethmosphserida (PI. 12, &c.). Many species of monozootic and poly- 

 zootic forms in all three groups are so alike that they can only be distinguished by the 

 fact that the one series are colonial, the others solitary. On the other hand, there are 

 some reasons which would justify a monophyletic hypothesis for the Polycyttaria, e.g., 

 the precocious nuclear division ; in this case it would be most natural to hold that the 

 Sphserozoida and Collos23ha3rida have arisen as two diverging branches from the CoUo- 

 zoida, whilst the latter are nothing else than colonial Thalassicollida. 



169. Phylogeny of the Acantharia. — The legion Acantharia or Actipylea is 

 distinguished by its peculiar acanthin skeleton, which develops centrogenously, as 

 well as by the disposition in groups of the pores in its central capsule, and its excentric 

 usually precocious nucleus ; it is thus so different from all other Eadiolaria as 

 undoubtedly to furnish, phylogenetically considered, an independent stem (§ 7). This 

 stem is only connected at the root by Actinelius with the primitive form of the Spumel- 

 LAKIA, Actissa. The stem is monophyletic, since all the forms belonging to it may be 

 derived without violence from Actinelius as a common primitive form. 



170. Origin of the Acantharia, — The genus Actinelius (p. 730, PI. 129, fig. 1), 

 which may naturally be regarded as the common primitive form of all Acantharia, 

 possesses a spherical central capsule, which in consequence of the early division of the 

 nucleus (§ 63), encloses numerous small nuclei ; from its centre arise many simple radial 

 spines of equal size, which penetrate the central capsule. A large number of radial 

 pseudopodia issue between the spines from the sarcomatrix which surrounds the capsule. 

 Actinelius may have been directly derived from Actissa, the common stem-form of all 

 Eadiolaria, by the division of the pseudopodia into two groups, myxopodia, which remained 

 soft, and axopodia, which became firm (§ 95a). As the latter became changed into strong 

 acanthin rods, and touched each other in the centre, they forced the nucleus from its 

 originally central position and brought about its early division. Actinelius is also of all 

 Eadiolaria the form which, next to Actissa, most nearly approaches the Heliozoa. If 

 the stiff axial threads of Actinosphcerium be conceived of as partially converted into 

 acanthin spines, and its nucleated medullary substance as separated from the alveolar 

 cortical layer by a membrane (central capsule), then Actinelius would be produced. 



171. Hypothetical Genealogical Tree of the Acantharia (see opposite page). 



172. Adelacantha and Icosacantha. — The numerous forms of Acantharia, here 

 disposed in twelve families and sixy-five genera, may be divided jahylogenetically into 

 two main groups of very different extent — Adelacantha and Icosacantha. The more 

 primitive group, Adelacaiitha, have an indefinite and variable number of radial spines. 



