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



less curved. In very few species only are they quite simple, without branches. 

 They are nearly always more or less branched, in many larger species very richly 

 ramified. The modes of ramification are rather variable. In the majority of 

 Plectoidea the spines are rather regularly verticillate, bearing an increasing number 

 of verticils, each of which is composed of three divergent branches. These arise from 

 the three edges of the spine, and all the branches of one edge are usually parallel, either 

 perpendicular to the spine, or directed at an acute angle towards its apex. When the 

 verticils are numerous (five to ten or more), their size commonly tapers gradually towards 

 the apex. Pinnate spines occur more rarely than verticillate ones ; in this case the two 

 paired lateral edges only of the prismatic spine bear opposite or alternate branches, 

 whilst the odd middle edge bears no ramules. In some species the spines are singly 

 or doubly forked. In many species (mainly those with cylindrical spines) the ramifica- 

 tion of the spines is more or less irregular. 



Whilst in all Plagonida the branches of the spines remain perfectly free, in all 

 Plectanida, again, the meeting ends of the branches become united and grow together, 

 and by this concrescence a loose network arises, like wickerwork, which partly encloses the 

 central capsule and the central parts of the spines, on which it rests. The meshes of 

 this loose wickerwork are large, either quite irregular, of very different size and form, or 

 more or less regular, with a certain form and arrangement of the meshes, effected by 

 the peculiar kind of ramification. Commonly the siliceous threads of the arachnoidal 

 wickerwork are very thin, often extremely delicate, representing " pseudopodia 

 metamorphosed into silex." Sometimes the wickerwork is spongy. Its surface is 

 constantly rough and bristly, with free ends of the spine -branches, never covered with 

 a regular lattice-plate, as in the Cyrtellaria (Spyroidea, Botryodea> and 

 Cyrtoidea). 



The entire form of the central wickerwork is in the minority of Plectanida quite 

 irregular and indefinite ; in the majority, however, a certain more or less regular entire 

 form is recognisable, effected by a certain, more or less regular origin and mode of 

 the connection of the meeting branches. So in some species of Triplecta (PI. 91, 

 fig. 7) the network represents a triangular plate, of Plectophora and Plectaniscus a 

 three-sided pyramid, of Tetraplecta (PI. 91, fig. 3) a tetrahedron, and in many other 

 species a polyhedron of more or less regular form. Some species of Plectanida become 

 very similar to certain species of Stephoidea, Spyroidea, and Cyrtoidea; 

 so Plectaniscus and Periplecta approach to Cortina and Cortiniscus, Pteroscenium 

 and Olathrocorys, &c. (compare Pis. 92, 93, 53, 64, &c.). They may represent a 

 true phylogenetic connection between both groups. But in these cases also the 

 distinction is determined by the fact that the true Plectoidea never possess a 

 complete sagittal ring (like the Stephoidea) nor a regular lattice-shell (like the 

 Spyroidea, Botryodea, and Cyrtoidea). 



