XVlll 



THE VOYAGE OF H.M.S. CHALLENGER. 



as " bilaterally symmetrical " in the strictest sense of the term. This is the most import- 

 ant ground-form in the animal kingdom, inasmuch as it obtains almost exclusively 

 among the higher animals (Vertebrata, Articulata, MoUusca, Vermes). The body con- 

 sists of only two antimeres, which correspond to the two symmetrical halves of the body. 

 Of the three dimensive axes two are allopolar, one isopolar ; the oral pole of the longi- 

 tudinal main axis is different from the aboral ; the dorsal pole of the sagittal axis is 

 different from the ventral ; but the right pole of the frontal axis is equal to the left. 

 The right antimere is usually precisely similar to the left (Eudipleura), more rarely it is 

 slightly dissimilar or asymmetrical (Dysdipleura). Among the Radiolaria this ground- 

 form is entirely wanting in the Porulosa or Holotrypasta (Spumellaria and Acan- 

 tharia), but on the contrary it is very common in the Osculosa or Merotrypasta 

 (Nassellaria and Ph^odaria). In the Nassellaria it is of special importance, for the 

 typical Cortina (the combination of the primary sagittal ring with the basal tripod) 

 exhibits the zygopleural ground-form clearly sketched out ; indeed it is usually clearly 

 seen even in the sagittal ring itself, for its ventral segment is more strongly curved than 

 the dorsal ; its basal (or oral) pole is always different from the apical (or aboral). Of the 

 three feet of the basal tripod the unpaired (caudal) one is directed dorsally and back- 

 wards, the two paired (pectoral) ones ventrally and forwards. The majority of the 

 Nassellaria may be regarded as modifications of this original ground-form. Its 

 relation to the primitively triradiate tripod presents a still unsolved problem, and the 

 numerous relations of the zygopleural to the multiradiate ground-forms in the 

 Nassellaria are exceedingly complicated. The zygopleural ground-form is less widely 

 distributed among the Ph^odaria, though it is very characteristically developed in 

 the rich and varied group of ChaUengerida (PI. 99). (See Gener. MorphoL, Bd. i. 

 pp. 507-527.) 



39. SynojJsis of the Geometrical Ground- Forms : — 



Geometrical Type. 

 1. Sphere, . 



Principal Groups of 

 Ground-Forms. 



Subsidiary Groups of 

 Ground-Forms. 



I. Centrostigma. 



The geometrical centre of 

 the body is a point. 

 Main axis wanting. 



I. Homaxonia. 



All axes equal. 



II. Polyaxonia. 



Endospherical polyhedra. 

 All the angles of the 

 body lie on the surface " 

 of a sphere. Numerous 

 isopolar axes. 



2. Endospherical polyhedron, 



3. Icosahedron, . 



4. Dodecahedron, 



5. Octahedron, . 



6. Cuhe, . 



7. Tetrahedron, 



Examples. 



Central capsule of the S p h ae- 

 r i d e a and of many AcAN- 



THARIA. 



Lattice-spheres of the S p h !e- 

 roidea, Sphierophracta, 

 and P h ae s p h as r i a. 



Circogonia. 



Circorrhcgma. 

 Cubosphserida, Circoporus. 

 Centrocuhus, Lithoctibits, kc. 

 Tetraplagia, Tetraplecta, &c. 



