REPOET ON THE TETRACTINELLIHA. 
Ixxxiii 
ARRANGEMENT AND DISTRIBUTION OE THE SPICULES. 
I. Microscleres. 
In Placina monoloplia, simplest of the Tetractinellida, microscleres of two orders 
of size present ; the smaller are monolophous microcalthrops, and these are distributed 
immediately beneath the epithelial surface of the sponge, chiefly occurring beneath the 
epithelium of the outer skin ; the larger are microcalthrops, microtriods, and microxeas, 
these are densely and uniformly distributed throughout the mesoderm, lying in 
contact with the flagellated chambers, to which their actines are tangential. A similar 
distribution occurs in Epallax callocyathus (q. v., Appendix, p. 423, PI. X. fig. 11). 
In Placina dilopha the lophose actines are obliquely directed outwards towards the 
external surface of the sponge ; in Placina trilopha the three lophose actines are directed 
towards the outer surface, so that this spicule is orientated like a trichotrisene, which it 
closely resembles ; in Thrombus challengeri the trichotrisenes are similarly directed when 
they lie near the exterior of the sponge, but in the choanosome they are dispersed without 
any approach to regularity. 
In most of the Theneidse microscleres of three orders of size are present, the smallest, 
spirasters or amphiasters, usually occur immediately beneath the epithelial surfaces, thus 
occupying a similar position to the smallest microscleres in Placina (occasionally they 
present forms suggestive of derivation from a lophose calthrops), those of intermediate 
size, the metasters, have much the same position ; the largest (plesiasters, euasters, or 
microxeas) are however no longer related to the flagellated chambers, but rather to the 
walls of the canals, to which their actines most frequently are tangential (PI. VII. fig. 2). 
In the remaining groups a relation between the orientation of the microsclere and the 
flagellated chambers is seldom to be traced. 
With the difierentiation of the sponge into ectosome and choanosome a differentiation 
of the microscleres is usually associated ; thus in the Tetillid, Chrotella macellata, ectosomal 
are to be distinguished from choanosomal microscleres; in the Stellettidse and Geodiidse, 
however, the chief distinction is into somal and choanosomal ; the microscleres which lie 
immediately beneath the epithelium of the cortex also occur throughout the choanosome, 
hence they are somal ; though they have so much the appearance of distinguishing the 
ectosome that by a slip {lapsus calami) they may sometimes be referred to as ectosomal. 
On the other hand, in the large majority of the Euastrosa a special variety of microsclere 
is confined to the choanosome, and this will be termed choanosomal. Sometimes in addition 
a special form of microsclere occurs both in the cortex and choanosome, but is restricted 
to the region surrounding the subcortical crypts ; these will be termed subcortical. 
In the Sterrastrosa the characteristic sterraster is present in addition to somal, choano- 
somal, and subcortical asters ; it occurs in all stages of development, scattered irregularly 
in the choanosome, but when adult it passes into the cortex, where it unites with its 
