REPOKT ON THE TETRACTINELLIDA. cxxiii 
4. A plagiotrisene, with a very much reduced rhabdome ; this spicule, which has been 
met with in one species only, is very similar to a calthrops {Tetilla merguiensis, Carter). 
5. Anatrisenes ; a somal form of this spicule can sometimes be distinguished from a 
radical form, — in the latter the cladome is usually grapnel-like, in the former hastate. 
6. Eeductions of both the protrisenes and the anatrisenes are far from infrequent, one 
or two of the eladi being suppressed, thus anadisenes, prodisenes, and monsenes result. 
Microscleres are usually present, though not always, and sometimes they are of more 
than one form ; if there is but a single form it is invariably a sigmaspire, if more than 
one, one of them is a sigmaspire. In two species the sigmaspire is spined {Tetilla japonica, 
Lampe, and Craniella atropurpurea, Carter) ; in one it is ceiitrotylote ( Tetilla geniculata, 
Marenzeller). The additional forms of microscleres are toxaspires, which occur in 
Chrotella macellata as the special microscleres of the cortex, those of the choanosome 
remaining as unmodified sigmaspires ; microstrongyles of variously curved forms, which 
occur in Tetilla stipitata. Carter, and microxeas, Tetilla australiensis, Carter. Minute 
globules are associated with the sigmaspires in several species both of Tetilla and 
Craniella. 
The number of different forms of megascleres present in the same species difiers 
greatly ; in some, e.g., Tetilla sanclalina, only oxeas and pro trisenes may occur, in others 
nearly all the forms which have been enumerated as occurring within the group. The 
somal megascleres are more or less closely associated together in bundles or fibres, which 
generally, if not always, radiate from a spicular centre or so-called nucleus, which is 
excentrically situated within the Sponge, from this they diverge in a more or less spiral 
course to the exterior. The cladomes of the trisenes do not attain their full growth till 
they reach or enter the ectosome. In some species of Tetilla the interspaces between the 
radial fibres are crossed by loosely scattered oxeas, not aggregated into fibres. 
The cortical megascleres, which are always oxeas, may be loosely and irregularly 
scattered through the cortex or more or less radially arranged {Craniella). 
The Ectosome. — In the simplest forms, such as Tetilla sandalina, there is a total 
absence of cortex, and the ectosome is a mere investing membrane, the choanosome with 
its flagellated chambers sometimes extending close up to the outer epithelium. From 
this stage the gradual evolution of the cortex is traceable in different directions, in Tetilla 
leptoderma and Tetilla grandis the ectosome is developed as a thin layer of fibro- 
vesicular collenchyma ; in Chrotella simplex this increases in thickness and becomes 
extensively excavated by intercortical cavities; corresponding with this advance we 
find a slightly greater development of fusiform cells in the inner layer of what may 
now be termed the cortex ; in Craniella, which completes the series in this direction, 
the inner layer of the cortex is clearly differentiated from the outer, the latter 
persisting as a collenchymatous tissue excavated by intercortical cavities, the former — 
converted into a fibrous tissue by the rich development within it of fusiform cells — 
