EEPORT ON THE TETEACTINELLIDA. 
109 
Hahitat. — Torquay ; Guernsey, at extreme low water (Bowerbank) ; Budleigli, 
Salterton, Devon (Carter) ; Westport Bay (Norman). 
Remarks. — The ectosome is not more than 0‘5 mm. thick ; in dried specimens I saw 
no appearance of fibrous tissue in it, it seems to consist of collenchyme, crowded with 
calthrops and pigmented granule-cells. The spinose microrabds are, when young, 
frequently even up to a length of 0'02 mm., smooth centrotylote microxeas, obviously 
diactinose asters. When sparsely spined they call to mind the microxeas of Ecionema 
hacillifera, Carter. They are generally scattered through the sponge, and form a thin 
layer bepeath the outer epithelium. The presence of the toxa is somewhat surprising, 
and Schmidt remarks that such spicules, characteristic as they are of the Desmacidine 
series, have, without any doubt, been merely accidentally introduced into the preparation 
[loc. cit., supra). This, however, is certainly not the case ; the toxas are as much parts 
of the sponge as any of the other spicules. It is not probable that any of the Pacha- 
strellidse have points of contact with the Desmacidina, and one would naturally look to 
ancestral forms such as Pcecillastra or Pachastrella for the homologue of this spicule. 
The microrabds can very well be explained as representing the microstrongyles of the 
latter or the microxeas of the former ; but the toxas, judging from analogy, are more 
likely to be traceable to a spiral than to an actinal form. The only spiral form in either 
Pcecillastra or Pachastrella is the spiraster, and in the absence of intermediate links, and 
considering the vast difference in size, it seems hazardous to suggest that this can have 
been the parent of the Dercitus toxa. In Caminus apiarium, 0. Schmidt, remarkable 
toxa-like spicules occur, evidently explicable as diactinose forms of an aster, but the 
toxas of Dercitus are not to be explained in this way ; they are more probably microxeas 
which have acquired a curvilinear growth. 
Norman in discussing the nomenclature of this sponge rightly claims priority for 
Dercitus over Pachastrella, but adds that undoubtedly Schmidt’s genus Pachastrella is 
identical with Dercitus, in this agreeing with Schmidt himself and Carter. It appears to 
me, however, that the differences between them are as great as in some other groups 
would serve to distinguish subfamilies. It is true that similar differences in different groups 
are of unequal value, but this is not always so. The only case in which it can be certainly 
alleged is when the differences are inconstant ; when they are constant but serve to 
divide a family into a number of genera, each containing a single species only, one cannot 
assert that they are of unequal value as compared to similar differences which in another 
family give us genera each with several species. The more natural explanation might 
then appear to be that a family with genera of single species is one which by severe 
exposure to unfavourable conditions has become impoverished. Such an explanation 
may possibly be true for the Pachastrellidse, the different genera of which are separated 
by characters which in the Stellettidse are of subfamily importance. 
