XCVl 
THE VOYAGE OF H.M.S. CHALLENGER. 
calcareous Sponges, and that as regards the most distinctively sponge-cells they possess, 
viz., the choanocytes. For, as Haeckel has shown, these are comparatively large in the 
Calcarea and small in the Plethospongise ; I have proposed, therefore, to use this 
character, which is more fundamental than that derived from the spicules, as a means 
of classification, naming those Sponges which are provided with comparatively large 
choanocytes Megamastictora (/xacrriAcrcup, o, the scourger, a fanciful term for a 
flagellated cell or choanocyte), and those with comparatively small choanocytes Micro- 
mastictora.^ The Megamastictora contain only a single subclass, the Calcarea, the 
Micromastictora are subdivided into three, the Hexactinellida, the Demospongise, and 
the Myxospongise. Lendenfeld,^ agreeing that the Sponges are subdivisible into two 
groups, which he terms subclasses, proposes to follow Gray, naming one Calcarea, 
and the other Silicea. 
The Megamastictora may be defined as Sponges in which the choanocytes are com- 
paratively large, from 0‘005 to 0'00.9 mm. in diameter, and in which the embryological 
development is marked by an amphiblastula stage. The Calcarea are Megamastictora 
possessing a skeleton of calcareous spicules. 
The Micromastictora are Sponges in which the choanocytes are comparatively small, 
not exceeding O'OOS mm. in diameter, an-d the embryological development is characterised 
by a blastula or planula stage. 
The subdivision of the Micromastictora is a subject on which there is general agree- 
ment among spongologists up to a certain point : thus it is generally admitted that the 
Hexactinellida form' a very natural group, which may be sharply separated from the rest 
of the class ; but a difference of opinion exists as to the most convenient mode of 
classifying the remainder, which constitutes by far the larger portion of the class ; 
Vosmaer and Lendenfeld propose two orders — the Spiculispongise and Cornucospongise, 
according to Vosmaer, or the Chondrospongise and Cornucospongiae, according to 
Lendenfeld. 
Vosmaer’s classification will be better understood by giving the following short 
abstract : — 
Class I. Porifera non-calcarea (equivalent to our Micromastictora). 
Order 1. Hyalospongiae (identical with the Hexactinellida : the term Hyalospongiae 
is a synonym). 
Order II. Spiculispongise. Skeleton rarely absent, when present consisting of 
independent spicules, which may be united by interlocking as in the 
Lithistida, or into a fibre by organic substance. 
Suborder I. Lithistina (identical with our Lithistida). 
1 Sollas, Zoological Record, vol. xxii., Spongiae, p. 13, 1886. 
2 Lendenfeld, Proc. Zool. Soc. Lond., 1886, p. 573 (1887). 
