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



by a second horizontal ring, the cortinar or basal ring (PI. 92, figs. 11-13). The pores 

 between the former and the latter, or the " cortinar pores," may be regarded as the first 

 beginning of the lattiee-j)late, composing the " cephalis " or the simple primordial shell 

 in the Archiperida and Tripospyrida, and transmitted from these by heredity to the 

 great majority of Nassellaeia. 



The "cephalis or capitulum " (the "Kopfchen" of the German authors) is therefore 

 the most important part of the skeleton in all C y r t e 1 1 a r i a, or in all Nassellaria 

 possessing a complete lattice-shell. In the Plectellaria, however, or in those 

 Nassellaria which do not j)ossess a complete fenestrated shell, the " cephalis " is either 

 imperfect or totally wanting. The cephalis surrounds the enclosed central capsule on 

 all sides in the form of an ovate or subspherical lattice -shell, and is separated from it only 

 by the jelly-like calymma. The sagittal ring is either enclosed in the wall of the cephalis 

 (wholly or partially), separating its two lateral halves, or it is enclosed in the cavity of 

 the cephalis and connected with its sagittal perimeter by short beams. The base of 

 the cei^halis (with the cortinar plate) often rests immediately upon the centre of the 

 tripod ; in the majority of Nassellaria, however, this near relation is altered by reason 

 of later changes and secondary modifications. 



The number of various forms, developed from these simple original structural elements 

 of the skeleton, is astonishing, and there are described more than three hundred genera and 

 nearly two thousand species of this legion in the following pages. This large number 

 may be easily increased by subsequent observers. Since in all these Monopylea the 

 characteristic structure of the central capsule is identical, and also the structural 

 elements of the siliceous skeleton are similar, it is very probable that they may 

 have arisen from a single common stock. But it is very difficult (and at present 

 impossible) to explain a natural monophyletic system of this large legion. The greatest 

 difficulty is presented by the fact that the three structural elements mentioned 

 above, viz., the sagittal ring, the basal tripod, and the latticed cephalis, are not 

 constantly united, but each alone may also constitute the skeleton by itself In this 

 respect the following seven cases are possible, and are actually realised. 



A. The skeleton is com^josed of the sagittal ring only and of its spiny appendages, 

 without basal tripod and without latticed cephalis. This is the case in the majority of 

 Stephoidea (Stephanida, Semantida, Coronida, and Tympanida). 



B. The skeleton is composed of a basal tripod only (Plagiacantha), or of a tripod 

 in the centre of which arises a vertical apical horn (Plagoniscus), and often of an 

 irregular framework, arising from the rods of the tripod; but there is neither a trace of 

 a sagittal ring nor a complete latticed cephalis. This is the case in the remarkable 

 suborder Plectoidea (Plagonida and Plectanida). 



C. The skeleton is composed of a latticed cephalis only, a simple ovate or sub- 

 spherical fenestrated shell, which encloses the monopylean central capsule; there is no 



