EEPORT ON THE TETRACTINELLIDA. 
Ivii 
Varieties of the Trisene. 
1. Anatrisene (andior of Carter) (Fig. X., ?z).— The cladi of the trisene are directed 
backwards soon after diverging from the origin. 
2. Protrisene (porrecto-ternate spicule of Bowerbank) (Fig. X., m). — The cladi are 
directed forwards, making an angle of less than 45°, with the axis of the rhabdome 
produced. 
3. Plagiotrisene (Fig. X., ^). — The cladi are directed forwards, making an angle of 
about 45°, with the axis of the rhabdome produced. 
4. Orthotrisene (Fig. X., 1 ). — The cladi make an angle of between 50° and 90°, with 
the axis of the rhabdome produced. Usually the angle approaches 90°. 
5. Dichotrisene (Fig. XL, a). — The cladi of a plagiotrisene or an orthotrisene are 
dichotomous. The protocladi are almost always directed forwards making an angle of less 
than 90° with the axis of the rhabdome produced, i.e., the dichotrisene usually arises from 
a plagiotrisene. In some few cases the protocladi are so highly porrectate'as to suggest a 
protrisene origin. 
6. Trichotrisene (PI. XXXI. fig. 10, a\ — A plagiotrisene or orthotrisene in which the 
cladi have become trifurcate. 
7. Phyllotrisene (PI. XXXII. figs. 8, 9). — The cladi of an orthotrisene, or dichotrisene, 
or trichotrisene may increase at the lateral margins in the plane of the cladome, and thus 
acquire a broadly expanded or lamellar form ; the margins of the foliate cladi thus 
produced are usually undulating or more or less divided. The phyllotrisene is only met 
with in the Lithistida. 
8. Discotrisene (PI. XXXI. fig. 5). — The cladome is a disc in which separate cladi 
are not distinguishable ; and the axial rods representing them extend but a short distance 
from the cladal origin. This spicule like the preceding occurs only in the Lithistida. 
The phyllotrisene and the discotrisene are of great interest as furnishing the best evidence 
of the mode of evolution of the Lithistid desma ; in the dichotrisene as in all normal 
spicules the axial rods of the cladi extend close up to the termination of the cladi, in 
the phyllotrisene they terminate at a distance from the ends of the cladi, greater or less 
as the case may be, in the discotrisene they terminate much nearer the origin, extending 
but a very short distance into the cladome, sometimes not more than 0'004 mm. Past 
the termination of the axial rods the disc grows by concentric additions to its margin, free 
from the control of the cladal axes. While the cladome thus follows the same course of 
growth as the desma of the choanosome, the rhabdome usually retains the normal spicular 
character, so that the discotrisene combines in itself the characters of the desma and the 
ordinary spicule. The essential character of the desma would thus appear to arise from 
emancipation from the control of the axial rods, which govern the growth of all other 
megascleres. 
(ZOOL. CHALL. EXP. PART LXIII. — 1888 .) 
Err li 
