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



collection of the Challenger, so that we may here distinguish eight genera and twenty- 

 seven species. 



The family Plectanida may be divided into four different subfamilies, according to 

 the number of the radial spines which compose the skeleton. These four subfamilies, 

 as also the genera contained in them, correspond exactly to the four subfamilies of 

 Plagonida, from which they have originated : Triplectida with three, Tetraplectida 

 with four, Hexaplectida with six, and Polyplectida with numerous (seven to nine or 

 more) radial spines. These spines are usually united (as in the corresponding 

 ancestral Plagonida) in one common central point, upon which rests the basal pole of 

 the central capsule, with the porochora. More rarely (in the genera Periplecta and 

 Plectanium) the spines arise in two opposite groups (each of two or three spines) from 

 the two poles of a common horizontal central rod, which supports the overlying 

 porochora of the central capsule. 



The ditiereut genera of Plectonida, which are enumerated in the following synopsis, 

 correspond so exactly to the different genera of Plagonida described above, that probably 

 each of the former has arisen from a corresponding genus of the latter. The only 

 difference between the two is, that in all Plagonida the branches and ramules of the radial 

 spines end freely, without growing together, whilst in all Plectanida the meeting ends of 

 the neighbouring spines grow together and so produce a loose and irregular wickerwork. 

 Therefore the latter constantly possess meshes, which are missing in the former. As 

 soon as any form of Plagonida begins to produce meshes by concrescence of meeting 

 branches of the spines, it passes over into a corresponding form of Plectanida. The 

 loose mckerwork or wattlework so produced is sometimes very irregular, at other times 

 more or less regiUar ; but it never assumes the regular form of a complete lattice -shell, 

 as in the C y r t e 1 1 a r i a (S p y r o i d e a, B o t r y o d e a, and C y r t o i d e a). The 

 general differences which this wickerwork exhibits in the different genera of Plectanida 

 have been already described above (compare pp. 900—904). 



The Central Capsule of the Plectanida agrees perfectly with that of the ancestral 

 Plagonida, as well in general form and structure as in the peculiar topographical relation 

 to the radial spines (compare above, p. 905). A slight difference between the two 

 families is effected by the higher development of the skeleton in the Plectanida. Since 

 the branches of the radial sj)ines in this family become united and form a loose wicker- 

 work, the central capsule becomes more or less enclosed by the latter, and more 

 perfectly protected, than in the Plagonida, where the branches remain free. In this 

 respect the Plectanida approach more nearly to the Cyrtoidea with which they are 

 connected by such transitional forms as Plectaniscus and Periplecta on the one hand, 

 Cladoscenium and Pterosceniiim on the other. 



