REPOET ON THE TETRACTINELLIDA. 
145 
Tlie anatrisenes likewise present, in the first observed stage, a long rhabdome expanded 
distally into a somewhat obconical thickening, from which three small spines project at 
the distal margin (PL XIII. fig. 20) ; they diverge at first horizontally, or at right angles 
to the rhabdome, and preserve this direction for some distance in specimens characterised 
by cladomes flattened in front; in those with rounded cladomes a slight backward 
curvature is often very early observed ; they subsequently grow backwards more than 
outwards, and so give rise to the adult form ; a further deposit of silica may quite 
conceal the originally horizontal growth of the cladi, converting an originally flat-fronted 
into a round-fronted cladome ; but the position and direction of the axial fibre remains 
unchanged, and faithfully records the successive change in direction of the growth of the 
cladi. The spicules of sponges grow outwards into the cortex, and are subsequently cast 
out from the sponge to be replaced by fresh ones ; evidence of this forward growth is 
afibrded by the sponge under description, since none but fully developed spicules are 
found with cladomes in the ectosome, and as we descend deeper into the sponge succes- 
sively younger forms are met with ready to replace those in front as soon as they are 
cast out. 
The large cells surrounding the spicules (PI. XIII. fig. 10), and serving apparently 
as mother-cells, are well seen in many of the sections of the sponge. They occur about 
the middle of the oxeate spicules, but about two-thirds from the proximal end in both 
forms of trisene. 
Anthastra parvispicula, Sollas (PI. XIII. figs. 30-40 ; PI. XL. figs. 1, 2). 
Antliastra parvispicula, Sollas, Prelim. Account, Sci. Proc. Roy. Dubl. Soc., voL v. p. 192, 1886. 
Sponge (PI. XIII. fig. 30), small, spherical, free, with a single small oscule. Pores in 
sieves, generally distributed. Surface even, soft to touch. 
Spicules. — I. Megascleres. 1. Oxea (PI. XIII. fig. 31), fusiform, usually curved,, 
somewhat sharply pointed, 1’3 by 0'02 mm. 
2. Orthotrisene (PI. XIII. fig. 32), a conical, sharply pointed rhabdome ; simple- 
conical cladi, diverging from the rhabdome at a wide angle, and rapidly, somewhat 
suddenly bending into horizontality. Rhabdome 1‘75 by 0’02 mm., cladi 0'21 mm. long. 
3. Anatrisene (PI. XIII. figs. 33, 34), a conical, sharply pointed rhabdome ; stout, 
short, conical cladi, rapidly recurving till nearly parallel with the rhabdome ; front of the 
cladome rounded. Rhabdome 1’3 by 0‘016 mm., cladi about 0'045 mm. long, chord 
0‘0485 mm., sagitta O'Oo mm. long, thickness of cladal rhabdome 0’025 mm. 
II. Microscleres. 4. Anthaster (PI. XIII. figs. 35-37), as in Anthastra pulchra, but 
with somewhat slenderer, less coarsely spined actines ; length of a single actine of a 
tetractinose form 0’014 mm., diameter 0'0035 mm. 
5. Chiaster (PI. XIII. fig. 38), a scarcely perceptible centrum and hair-like actines 
abruptly truncated, not tylote; diameter 0'0118 mm. 
(ZOOL. CHALL. EXP. — PART LXIII. — 1887.) 
Err 19 
