150 
THE VOYAGE OF H.M.S. CHALLENGEE. 
Genus 5. Stelletta, 0. Sch. 
Oscules may or may not be evident. Pores in sieves overlying completely differen- 
tiated choues. Cortex well developed, consisting of an outer collencbymatous layer 
passing externally into a thin fibrous layer, and of a clearly differentiated thick inner 
fibrous layer. Chones completely differentiated. Of the two forms of aster, one is 
characteristic of the ectosome and the other of the choanosome, though neither is 
confined to the region it characterises. The ectosomal asters, however, form a layer 
beneath the outer epithelium, from which the choanosomal aster is rigidly excluded. 
Stelletta 'plirissens^ Sollas (PL XVI. figs. 6-20). 
Stelletta plirissens, Sollas, Prelim. Account, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 192, 1886. 
Sponge (PI. XVI. figs. 6, 7) spherical to cylindrical, compressed, with rounded edges ; 
attached. Surface bristling with spicules which project for a distance of 6 to 7 mm.; 
embossed, a polygonal network of linear depressions defining low rounded swe llin gs or 
bosses, about 2 to 3 mm. in diameter. Pores in sieves, chiefly distributed over the linear 
depressions and the sides of the bosses. Oscules small, not exceeding 1 mm. in diameter, 
distributed in scattered groups. Cortex thick, consisting of an outer collenchymatous 
and an inner fibrous layer without special spicules, except for a single layer of minute 
spherasters occurring immediately below the outer epithelium. 
Spicules. — I. Megascleres. 1. Oxea, fusiform, obtusely pointed, 4‘65 by 0'073 to 
4'89 by 0'07 mm. 
2. Dichotrisene (PI. XVI. fig. 8); a stout conical rhabdome, below the cladome faintly 
constricted, then for a short distance cylindrical or bulging, finally attenuating, till when 
about 0'0125 mm. in diameter it ends in a rounded point ; protocladi curving forwards 
and outwards, cyathiform ; deuterocladi conical, bluntly pointed, diverging horizontally. 
Ehabdome from 3’5 by 0‘118 to 4T9 by 0T2 mm., protocladi from 0T27 to 0T43 mm. 
long, about half the length of the deuterocladi, which are about 0‘2o4 to 0'303 mm. long, 
chord of the cladome 0‘7 mm., of the deuterocladi 0‘286 mm. 
3. Anatriasne (PI. XVI. fig. 9) a somewhat fusiform rhabdome, first diminishing in 
thickness from the cladome proximally, then increasing, and finally diminishing, till when 
about 0'008 mm. in diameter, it ends in a rounded point; cladi conical, with obtuse or 
rounded points, gently curving outwards and scarcely backwards for one-third of their 
length, then somewhat suddenly bent more backwards than outwards ; cladome slightly 
projecting in front. Ehabdome from 8'72 to 8’1 mm. long; 0‘051 wide below the head, 
diminishing to 0‘037, and then increasing to 0‘058, finally attenuating to 0'008 mm. in 
diameter, and endihg in a rounded point. 
^ (p^iaaa, to bristle. 
