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



usually differeut), more rarely the frontal axis (with equal right and left poles). In the 

 zygothakmous S p y r o i d e a (as in the S t e p h o i d e a) the formation of the shell proceeds 

 from the sagittal ring, whilst in the polythalamous C y r t o i d e a the latticed cephalis 

 is always the starting point, from which a series of joints (thorax, abdomen, and in the 

 Stichocyrtida, the numerous post-abdominal joints) successively arise (unipolar growth). 



149. Tlie Ontogeny of the Phceodaria. — The individual development of the Ph^odaria 

 in the simplest case stops with the skeletonless condition of the Phseodinida {Phceodina, 

 Phceocolla), which can be immediately derived from the foregoing Actissa-st&g& by the 

 disappearance of the pores in the greater part of the central capsule, the characteristic 

 astropyle being developed at the basal pole (§ 60). Since this particular form and 

 structure of the spheroidal central capsule remains the same in all Phceodaria, whilst 

 the formation of their skeleton follows very different directions, it follows that further 

 common paths of development are excluded both ontogenetically and phylogenetically. 

 What will be laid down in this respect as regards the phylogenj^ of the different groups 

 of PniEODARiA (§§ 194-199) holds true also of their ontogeny. The relations of growth 

 in the three dimensive axes are hence very different in the skeletons of the various 

 groups of Ph.eodaria. This difference is best marked in the Phseoconchia, whose 

 bivalved lattice-shells have as their ground-form the rhomboid pyramid of Ctenophora. 

 In most Phseogromia the monaxon lattice-shell may develop simultaneously by 

 sudden excretion at a j^articular moment of lorication ; this is also the case with the 

 polyaxon lattice-shells of the P h Ee o s p h ee r i a. In their further growth the develop- 

 ment of basal or radial apophyses is of special importance. In the majority of the 

 Phceodaria these apophyses are tubes of silicate filled with jelly (often provided with 

 an axial siliceous thread) ; thus their development is distinguished by complications 

 which are absent in the case of the other three legions. 



"O 



150. Growth. — The growth of the body in the Radiolaria, as in all other organisms, 

 is the fundamental function of individual development (see note A). All structural 

 relations which this richest class of the Protista exhibits may be referred to different 

 forms of growth, either of the unicellular malacoma or of the skeleton which it produces. 

 In general the special development of the skeleton is dependent upon that of the central 

 capsule, and of the sarcodictyum on the sui-face of the calymma ; in the further growth, 

 however, the conditions are reversed, and the condition of the skeleton already formed 

 directly determines the further development of the central capsule and of the calymma 

 with its sarcodictyum. The four legions of Radiolaria show, speaking generally, 

 certain characteristic differences in growth, which are due in great part to the different 

 structure and ground-form of their central capsule. In the two legions of the Porulosa 

 (Spumellaria and Acantharia), in which the central capsule is originally spherical and 



