52 THE VOYAGE OF H.M.S. CHALLENGEK. 



manner into a continuous framework. This fusion is effected by a process of cementing 

 (soldering), or by means of synapticula. 



Tlie dermal skeleton is formed of sword-like hypodermal hexacts, of which the pro- 

 longed proximal rays penetrate the parenchyma at right angles to the surface, while the 

 shorter distal rays raise the outer skin into a conical point, and either bear on their tips a 

 protruding floricome, or are surrounded by several freely projecting pointed diacts. The 

 four tangential rays of the hypodermalia, which stand at right angles to one another, lie 

 just below the sieve-like perforated skin, and form by regular apposition of the corre- 

 sponding rays a quadrate network. 



The gastral skeleton is similarly formed of hypogastral hexacts or pentacts. The 

 prolonged distal ray enters the parenchyma at right angles to the gastral surface, while the 

 opposite proximal ray, when fully developed, raises the gastral skin in an internal point, 

 and also, in some cases, bears on its end a floricome. By the closely apposed tangential 

 rays of the hypodermalia a quadrate lattice-work is formed for the support of the gastral 

 skin. 



In. the parenchyma of the body, in addition to simple hexacts and their derivatives 

 w^th a smaller number of rays, numerous rosettes occur, chiefly oxyhexasters and 

 discohexasters. 



The chambers, which lie close to one another in a very much folded single layer, have 

 a simple saccular or beehive form. In their thin walls can be seen the elegant quadrate 

 network formed from the anastomoses of the ciliated cells, and here and there a round 

 chamber pore. The delicate trabecular framework, which extends on the one hand between 

 the outer skin and the chamber layer, and on the other, between the latter and the gastral 

 membrane, is abundantly penetrated by the lacunse and canal-like apertures of the in- 

 current and excurrent canal system ; but besides this, there extends everywhere, between 

 the openings of the chamliers, a thin net-like layer, in which numerous oxyhexasters Lie 

 embedded. 



According to the presence or absence of parietal gaps, I have established two sub- 

 families, namely, the Euplectellinse with, and the Holascinae without gaps. 



Subfamily 1. Euplectellin^ (Pis. I.-VL; PL XIII. ; PI. XIV. figs. 1-5). 



Euplectellidse, in which the lateral wall is perforated either by more or less regularly 

 arranged circular gaps, with a membranous margin and circular muscles, or by iri-egular 

 angular apertures. The sword-like hexacthypodermalia bear a floricome at the end of 

 their distal rays. The hypogastralia are, as a rule, simple pentacts without a proximal 

 ray, but here and there hexacth}q30gastralia also occur, with a floricome at the end of 

 the principal ray. The central tj^a and best starting-point for the study of this 

 subfamily is the genus Eujilectella. 



