REPORT ON THE TETRACTINELLIDA. 
XCV 
Subkingdom PARAZOA. 
Heterocytal organisms in which the endoderm, in whole or in part, consists of 
choanocytes. 
Phylum SPONGIA. 
Parazoa possessing a paragastric cavity, which communicates with the surrounding 
medium by means of pores. 
Subdivision of the Phylum into Classes. 
The subdivision of the phylum which commends itself as the most natural is into 
two classes, the one containing the Sponges that are provided with calcareous spicules 
and the other those that are not. This broad distinction was first made by Gray,^ who 
named the two groups Porifera calcarea and Porifera silicea ; Vosmaer^ was the first to 
recognise the justness of this arrangement, but rightly objecting that the term “ Silicea” 
is misleading, and recognising the difficulty of finding any positive character by which 
the class could be known, proposed to substitute for it “ Non-calcarea”; Polejaeff,® in his 
luminous Eeport on the Challenger Calcarea, recognises the independence of the Calcarea 
as a class opposed to all other Sponges, but does not attempt to find a name for the 
latter. I have proposed^ to name the non-calcareous Sponges Plethospongia, a term 
which is not open to the objection of disjointedness, but which does not express any 
definite meaning. The difficulty of finding a term which shall, in an epigram of a 
single word, designate the group, arises from the fact that, while the Plethospongia are 
an eminently natural class, yet the characters by which they are united together are 
not common to all the members, but change from family to family, so that their union 
is a linked one, and while the families at opposite ends of the chain appear to differ 
toto coelo, they are united together by a continuous series of intermediate forms. It is 
possible that this difficulty arises from the fact that in the consideration of the Pletho- 
spongia the skeletal structures have been too exclusively regarded, and our knowledge 
of the soft parts is not yet extensive enough to enable us, with anything like confidence, 
to make use of their characters in classification ; yet there is one distinction which 
obtains at least very generally, if not universally, between the calcareous and non- 
1 Gray, Proc. Zool. Soc. Land., p. 502, 1887. 
2 Vosmaer, Report on the Sponges dredged by the “ Willem Barents ” in 1878 and 1879, Nied. Arch. f. Zool., SuppL- 
Bd. i., 1882. 
^ Polejaeff, Report on the Calcarea, Zool. Chall. Exp., 1883, part xxiv. p. 22. 
^ Sollas, Sci. Proc. Boy. Dull. Soc., vol. v., N.S., p. 112, 1886; an abstract of this note appeared in 1885, 
Ann. and Mag. Nat. Hist., ser. 5, vol. xvi. p. 395. 
