EEPORT ON THE TETRACTINELLIDA. 
lix 
The Rhabdocrepid Desma. 
Those megascleres of the Lithistida that form the characteristic skeletal network 
differ in some important respects from the megascleres of other sponges, and are here 
distinguished as “ desmas ” ^ (Secrp.a, aros, to, a bond). They are formed usually 
by the deposition of successive layers of silica upon an ordinary spicule, the axial rod 
of which early suffers an arrest of development (PI. XXXIII. fig. 8 ; PI. X XX IV. 
fig. 1 ; PI. X XX V. figs. 5—10, 35—42). The layers which are deposited after the 
arrested growth of the fundamental spicule, or, as it may be termed, the “crepis” 
{Kp'qTTis, tSo 9 , j), a kind of man’s boot, or generally a foundation), are at first concentric 
with it, but subsequently grow out into irregular branches, cladi, and tubercles which 
are altogether independent of it. The crepis may be either a monaxon (Fig. X,, p—r), 
or a tetraxon ; the former, which alone is the immediate subject for consideration, is 
most usually if not always a small strongyle (microstrongyle). That part of the adult 
desma which is formed by the deposition of concentric layers immediately around the 
crepis may be termed the “ epirabd.” The fully grown forms of a rhabdocrepid desma 
are described under the species in which they occur, and we need not further allude 
to them here except to point out the fact that in many cases examples of rhabdo- 
crepid desmas will frequently be encountered which, notwithstanding their difference 
in origin, are not to be distinguished from normal forms of tetracrepid desmas, and 
thus it may be and undoubtedly is in some cases difficult to say, in the absence of a 
visible crepis, whether a desma is rhabdocrepid or tetracrepid, but this difficulty is not a 
matter of any consequence in the examination of recent species, since, if the crepis is not 
visible in one desma it will be in another ; it is only in the fossil sponges that any real 
difficulty can occur, and here it may generally be met by the fact that while some 
rhabdocrepid desmas are remarkably like tetracrepid ones, the converse on the other 
hand is not generally true, and thus if all the desmas of a fossil sponge are tetracrepid 
in general appearance, we may assign it to the tetracrepid group of Lithistids with great 
probability, even when neither the crepis is visible nor its cast ; on the other hand, if 
only a few of the desmas are tetracrepid in appearance and the rest rhabdocrepid, we 
may refer the sponge to the rhabdocrepid division, undisturbed by the few exceptional 
forms which appear to point to a different conclusion. 
Group II. Classification of the Teteaxons. 
1. Tetractine.— —When all four actines of a tetraxon are present it is of course a 
tetractine, but as the full designation of this required to distinguish it from a tetractinose 
1 Of course the plural form should be “desmata,” hut in this and all similar cases (dragma, sigma) I have ventured 
to form the plural according to the common English rule. 
