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



(Botryopylida and Botryocyrtida) are to be regarded as primitive ; the Botryocellida 

 and Botryocampida have arisen by the closure of this mouth with a b asal lattice- 

 plate. 



190. Genealogical Tree of the Cyrtoidea. The multiform and extensive group 

 Cyrtoidea presents the greatest difficulties to be found in the phylogeny of the 

 NASSELLAEIA, because their morphological relations are most complicated, and because 

 similar forms very often appear to be of quite different origin. The great majority of 

 the Cyrtoidea show more or less clearly a combination of the three structural elements; 

 sagittal ring, basal tripod, and latticed cephalis (p. 891). There are also, however, 

 numerous Cyrtoidea, whose skeleton no longer shows any trace of the sagittal ring. 

 Many of these show as the basis of the skeleton a strong basal tripod with an apical 

 spine, around which the cephalis has obviously been secondarily developed, e.g., the 

 remarkable Euscenida (p. 1146, Pis. 53, 97) and the interesting Callimitrida (p. 1217, 

 Pis. 63, 64). These may have been derived immediately from the Plectoidea 

 without any relation to the Stephoidea. There are also numerous true Monocyrtida, 

 whose shell consists of a simple latticed cephalis without a trace of the sagittal ring or 

 basal tripod (Cyrtocalpida, PI. 51, figs. 913 ; PI. 98, fig. 13); these may have been 

 developed directly from the skeletonless Nassellida by the formation of a simple ovoid 

 Gromia-like shell, and may have no relation either to the Stephoidea or 

 Plectoidea. On these grounds, as well as from the complicated relationships of 

 the many smaller groups of C y r t o i d e a, it is probable that the whole order has been 

 developed polyphyletically from different divisions of the Plectellaria. 



191. Systematic Arrangement of the Cyrtoidea. Although for the reasons just 

 given no systematic arrangement of the Cyrtoidea can at present, or for a long 

 time in the future, be regarded as other than artificial, yet some general principles of 

 classification for this extensive group can be laid down, which may serve as starting 

 points for some future natural disposition. This is especially true of the relations 

 which in an artificial system (p. 1129) were primarily utilised for the distinction of 

 twelve families and twenty -four subfamilies ; the number of segments in the shell, the 

 number of radial apophyses (and parameres), and the constitution of the basal aperture 

 of the shell. 



As regards the number of segments, separated by transverse constrictions, of which the shell is 

 composed, it is dependent upon the secondary addition of new joints at the basal pole of the main 

 axis. Hence all many-jointed Cyrtoidea are to be derived from single-jointed ones, and the 

 four sections thus distinguished (Monocyrtida, Dicyrtida, Tricyrtida, Stichocyrtida) form a phylo- 

 genetic series. Very often, however, the primary cephalis disappears owing to retrograde metamor- 

 phosis ; and in such cases the single joint of the apparent Monocyrtida is formed of the thorax (e.g., 



