EEPOET ON THE TETRACTINELLIDA. 
Ixxxi 
opposing surface, and in some cases becomes terminated by a tylus, as in the oxytylotes 
of Esperia marshalli, and as was probably the case with the ancestral form of trieene, 
since these spicules although they commence to develop a tylus while situated in the 
interior of the sponge do so possibly by precocity. If not, we may suppose that 
tangential strain in the interior of the sponge leads to a general terminal growth. As 
the tylus which we suppose to have formed immediately below the skin increases in size, 
it grows along three lines of least resistance inclined to each other at angles of 120°, and 
thus the trisene results. 
Again, take the case of a simple calcareous sponge : let a scleroblast be situated near 
the surface of the sponge, as it must be in the Ascones ; the surface tension will here also 
lead to the growth of three actines inclined at angles of 120° to each other, and thus the 
triradiate spicule so common in the calcareous sponges may have arisen. 
Eeturning to the trisene, the growth of the cladi may continue in a straight line, or 
bifurcation may take place, and if it does the deuterocladi should, according to theory, 
make angles of 120° with each other and with the protocladus, or if not, the angles 
between the protocladus and each of its deuterocladi should be equal. Observation here 
supports theory, these conditions, one or other of them, being always fulfilled in the case 
of the dichotrisenes. 
Several matters of detail remain for discussion ; in the first place the form of the 
protrisenes in the Tetillidse, and of the early forms of orthotrisenes in the Stellettidse and 
other Tetractinellida, cannot be lightly passed over. The last-named spicules at the time 
they appear in the choanosome being practically protrisenes, are susceptible of the same 
explanation as seems inevitable in the case of these spicules in the Tetillidm, i.e., we 
must suppose that they were evolved under the actions of tensions which are the 
resultants of the radial and tangential tensions ; given that the cladi lie in the direction 
of the resultant, and it is possible to determine the ratio between the radial and 
tangential forces which have determined their direction, for in a triangle of forces the 
length of the sagitta will represent the magnitude of the radial tension, and half the 
length of the chord that of the tangential tension ; in a very young specimen of Crani- 
ella sclimidtii, still enclosed within the body of the parent, I find by measurement that 
the radial is to the tangential tension as three to two. In the Tetillidse the protrisene 
retains its protrisene form throughout life, but in the Stellettidse and Geodiidse it subse- 
quently passes into a plagio-, ortho-, or dicho-trisene. This change, in the case of young 
spicules developing in a fully grown sponge, takes place as they approach the outer 
epithelial surface, towards which they travel as they grow, and the tangential direction of 
the cladi is not fully assumed till they lie quite close beneath the skin or the floor of 
an intercortical cavity, where we may fairly assume that the tangential tensions are 
at a maximum. This change in the direction of the cladi is frequently very marked, 
especially in the case of the dichotrisenes, in which the protocladi may be directed like 
(ZOOL. CHALL. EXP. — PART LXlII. — 1888.) Err I 
