REPORT ON THE TETRACTINELLIDA. 
259 
(No. 2) and occasional anatrisenes ; a sterrastral layer 0'8 mm. thick ; and an inner fibrous 
layer 0'08 mm. thick. 
Spicules. — I. Megascleres. 1. Somal oxea, S'57 by 0'45 mm. (N.) and 3'2 by 0’055 
mm. (H.). 2. Cortical oxea, 0'45 by 0*004 mm. (H.). 3. Orthotrisene, rhabdome 3*38 
by 0*042 mm. (N.) and 3*14 by 0*067 mm., cladi 0*366 mm. long (H.). 4. Protrisene, 
rhabdome 0 006 mm. in diameter, cladi 0*1 mm. in length (N.), rhabdome 5*7 by 0*02 mm., 
cladi 0*118 mm. in length (H.). 5. Anatrisene, rhabdome 0*0194 mm. in diameter, cladi 0*1 
mm., chord 0*13 mm. long (N.), rhabdome 6*6 by 0*0276 mm., cladi 0*126 mm., chord 
0*13 mm., sagitta 0*13 mm. long (H.). 
II. Microscleres. 6. Sterraster, spherical, 0*077 mm. (N.), 0*067 mm. (H.), in 
diameter. 7. Somal chiaster, actines short, cylindrical, truncate or tylote, 0*007 mm. 
in diameter. 8. Choanosomal chiaster, actines slender, conical, usually truncate, some- 
times roughened near the ends, 0*04 mm. in diameter. 
Colour. — Yellowish -grey. 
Habitat. — Adriatic. 
Remarks. — The letter (N.) indicates that the measurements were taken from a frag- 
ment of Geodia gigas, presented to Dr. Norman by 0. Schmidt, (H.) from a whole 
specimen obtained by Professor Haddon from the Zoological Station, Naples. The two 
sets of measurements are fairly accordant. 
Amongst the hispidating oxeas I observed some very slender anatrisenes with minute 
cladomes, the chord not measuring above 0*007 mm. The appearance of these small 
spicules, where we usually meet with the most completely developed forms, was so 
unexpected that it naturally arrested attention and led to further investigation, which 
was rewarded by finding that the small cortical oxeas sometimes bear cladi like those of 
the anatrisene, and in one instance a typical fusiform oxea, only 0*394 by 0*004 mm. in 
size, was seen, bearing at its distal pointed end a true anatrisene cladome, with a chord 
only 0*004 mm. long. The resemblance of these minute anatrisenes to the cladose tylo- 
styles of Proteleia sollasi, Ridley and Dendy, is a very suggestive fact. Between the last 
named sponge and Cydonium gigas there can hardly be any close connection, and if we 
cannot regard the cladose spicules of either as directly descended from those of the other, 
we are led to conclude that we have here a new and striking case of homoplastic develop- 
ment. The hispidating oxeas share with the other spicules of the sponge a molecular 
structure which is similarly plastic to the influence of tangential strains in the ectosome, 
leading to a triradiate branching near the ends. 
