REPORT ON THE TETRACTINELLIDA. 
311 
is real enough ; I doubt whether any one, including Professor Zittel himself, could 
distinguish the dichotrimnes of some Lithistida from those of some Choristida, even 
under the highest powers of the microscope. In the general introduction we have 
shown reasons for regarding the Lithistida as derived from the Choristida, and not 
vice versd, and connected with this is the probability that the dichotrisene is a part 
of the Lithistid inheritance from the Choristida. Thus then we may regard the 
discotrisene as a modified dichotrisene, and not vice versd. If now we consider the 
disc, it may have been derived from the discotriaene, or independently evolved. Thus 
in ScleritoclermO; to which the suggestion of being a parental, rather than a filial form 
is given by the presence of sigmaspires, we find strongyles very similar to the crepides 
of the monocrepidial desma accumulated to form a subdermal skeletal layer. The 
tendency amongst the Lithistida for spicular structures lying near the surface of the 
sponge to grow out in a plane parallel to it is exemplified by the broadening of 
the cladi of the dichotrisene, by which it becomes converted into the discotrisene. If 
the same tendency should affect the strongyles lying beneath the epithelium in Sclerito- 
derma, they would become converted into discs like those of Neopelta ; if some of these 
strongyles lie obliquely not quite parallel to the surface of the sponge, we may expect 
the distal end alone to expand, and then the discs with oblique stalks, also characteristic 
of Neopelta, would arise. Thus a separate origin for the Neopelta-di&c is quite conceiv- 
able, and the explanation just given accounts for the position of the crepidial axis in the 
plane of the disc, a feature very difficult to understand on the hypothesis that the discs 
are modified discotrisenes. 
But next we have to consider the relationship of the discostrongyle of Callipelta ; 
in the fact that the shaft is nearly always directed at right angles to the disc, and that 
the crepidial axis never lies in the plane of the disc, even when the shaft is absent, this 
much more closely resembles a discotriaene than a disc ; the desmas of Ccdlipelta are also 
much more like those of Corallistes than of Neopelta, and I think we may with greater 
probability regard the discostrongyle as a reduced discotriaene than as a modified disc. 
This being so the issue is much narrowed, and the only point for inquiry which remains 
is as to the relation of the discotriaene series (inclusive of the discostrongyle) to the 
Neopelta-di\s,c„ Is the latter a reduced discostrongyle, or of separate origin, or is any 
third explanation possible ? I must confess that to me it is difficult to answer the first 
part of the question in the affirmative, an affirmative to the second one would avoid if 
one could, and a third explanation may be suggested. Instead of regarding the com- 
pleted disc, let us fix our attention on the crepis ; in the,dichotriaene this is of course a 
dichotriaene, in the discotriaene it is frequently reduced to an orthotriaeue ; the general 
superficial extension of the cladome being independent of the direction of the deuterocladal 
axes, the existence of these ceases to have any meaning, and they disappear as useless 
structures ; but the protocladal axes are also useless, and they similarly but subsequently 
