EEPOET ON THE TETEACTINELLIDA. 
157 
chambers measure about 0’0316 mm. in diameter; they communicate with the incurrent 
canals by wide prosopyles or short prosodal canals, and with the excurrent system by 
aphodal canals of no great length. 
Skeleton . — As in other spherical Stellettids the megascleres are arranged in radial 
fibres proceeding from the centre outwards. Trisenes first occur in the choanosome in 
the immediate vicinity of the cortex ; here the cladomes of young anatrisenes, and of 
orthotrisenes, now in a protrisene stage, are first seen. The adult trisenes extend their 
cladi within the cortex, at various levels, the orthotrisenes mapping out the overlying 
epithelium into intercladal areas in which the pores are situate. Associated with the 
outer epithelium, coating its inner surface, a dense layer of sanidasters occurs ; less 
numerously, but still abundantly, these microscleres occur scattered throughout the 
cortex, particularly below the epithelium of the intercortical canals ; within the choano- 
some they are sparingly distributed. 
The Cloacal Tube — Skeleton.— TliQ megascleres of the cloacal tube consist wholly of 
orthodisenes collected in parallel, more or less equidistant, fibres which extend the whole 
length of the tube. Each fibre consists of several spicules lying side by side, others 
overlapping them at the ends, the overlap taking place in such a manner that the cladal 
end of the one spicule is always exterior to the oxeate end of the other, in other words, 
the spicules are imbricated with the cladomes outwards. The cladal ends of the spicules 
are directed distally, the cladomes, lying immediately beneath the outer epithelium and 
its associated tissue, succeed one another in the same fibre at more or less regular intervals, 
the cladal centres of succeeding spicules alternating with more or less regularity on each 
side of a medial line. The cladi of adjacent or peenadjacent^ fibres are given off at about 
the same levels, and as each cladus extends across the space between three fibres or even 
further, the cladi of neighbouring fibres overlap. And further, since the cladi make right 
angles with the rhabdomes, the foregoing arrangement leads to the formation of a rect- 
angular spicular framework, with interspicular rectangular spaces (PI. XLI. fig. 5). The 
regularity of this arrangement is disturbed in various ways ; the longitudinal fibres 
occasionally branch, and the interval between the branch and the main fibre, at first very 
narrow, increases till it reaches the average ; thus not only exceptionally narrow areas 
are produced, but areas with diverging longitudinal sides. The cladi of adjacent fibres 
are not always given off at the same level, and thus the interspicular rectangles may 
become transversely subdivided into two or more of less than the average length. 
The spicules are more numerous in each fibre, and the cladi arise at closer intervals as 
one approaches the origin of the tube ; near its distal extremity there may not be more 
than two or three spicules to a fibre, and the cladi are as often as not separated by 
intervals of from 0'75 mm. to nearly I'O mm., while in the middle of the tube the inter- 
cladal intervals are about 0’08 to 0*3 mm. long, and on average about 0'24 mm. 
1 We have “ paenultimate,” why not “paenadjacent”? 
