312 
THE VOYAGE OF H.M.S. CHALLENGER. 
disappear, they are of very variable size even in the spicules in which they are constantly 
present, frequently reduced to 0'004 mm. in length, and their disappearance as in the 
discostrongyle need cause us no surprise. A reduction, in this case more readily intelligible 
than in many others, has taken place of a precisely similar character to that which pro- 
duces a microxea from a spiraster or an oxyaster. The crepis, now become monaxial, 
is liable to an independent series of modifications, and no longer compelled into a 
definitely orientated position by the presence of cladal axes, assumes a position partly 
tangential, partly radial, and partly intermediate or oblique, the disc-like expansion is 
produced by the same tendency as existed before, and discs result in which the crepis may 
lie either wholly immersed, or only partially, as in Neopelta. But the existence of the 
discs themselves is no longer necessary with the changed position of the crepides, and so 
in Scleritoderma we find lying beneath the epithelium merely a felt of strongyles alto- 
gether tangentially arranged. Thus owing to the changes in the structure of the adult 
spicule working their way backwards till they affect the embryonic spicule or crepis, 
fresh series of changes are evoked which lead to fresh series of modifications in the adult. 
This hypothesis serves to connect the dichotrisenes of Corallistes with the strongyles of 
Scleritoderma ; but it is purely a hypothesis, and in view of the presence of sigmaspires 
in the last-named genus, the possibility that it may be an advancing and not a reduced 
form must be carefully borne in mind. 
Family III. Pleromida:. 
Trisenophora in which the desma is monocrepid, and smooth, not tuberculated ; 
zygosis is produced by the expanded ends of the cladi of one desma clasping the sides of 
the epirabd or cladi of another. 
Genus 1. Pleroma^ n. gen. 
Pleromidae in which the flagellated chambers are large, with wide, short aphodi. The 
microscleres are microxeas and spirasters. 
Pleroma turhinatum, n. sp. (PL XXXIII.). 
Sponge (PI. XXXIII. figs. 1-2). — Small, obconic, compressed, terminating below in a 
small, short, rounded pedicel, upper surface slightly convex, depressed in the middle, 
margin rounded, oscules small, confined to the upper surface, the simple openings of 
narrow vertical canals. Pores simple, generally distributed over the sides. 
Spicules. — I. Megascleres. 1. Desma (PI. XXXIII. figs. 7-7e) ; this consists of a 
1 vT^yipufiec, ro, a full measure, complement; in allusion to the presence of a full complement of spicules. 
