Ixxxiv 
THE VOYAGE OE H.M.S. CHALLENGER. 
fellows by fibrillar fusiform cells to form a special layer — the sterrastral layer of the cortex. 
Considering the large size of the sterraster and the important part it plays in contributing 
to the skeleton of the sponge the sterraster might claim to be classed among the megascleres. 
II. Megascleres. 
Two types of skeleton may be distinguished in the Tetractinellida, the radiate and 
the irregular. The irregular is met with in Placinastrella copiosa, F. E. S., the genus 
Pcecillastra and its allies, in the PachastreUidae, and the Lithistida ; but in aU cases, 
triaenes, when present, lie near the surface of the sponge orientated as in the radiate type, 
^.e., with the cladome tangential to the surface and the rhabdome directed radially inwards ; 
this is the nearest approach to regularity found in this group. In the choanosome of the 
Pachastrellidae and Placinastrella the calthrops are scattered without order, though some- 
times the actines tend to lie tangentially to the walls of the canals ; in some species of 
Pachastrellidae long slender rhabdi are present, and these often run in fibre-hke tracts, which 
near the surface of the sponge are directed at right angles to it ; one actine of a calthrops 
also may frequently be observed directed in parallelism with the rhabdi of such fibres. 
In Pcecillastra, groups of oxeas arranged in parallelism and calthrops with one actine 
parallel to the oxeas form the greater part of the skeleton ; the spicule “drift” takes two 
chief directions, one more or less parallel to the chief direction of growth, lying in a plane 
parallel to the face of the plate-like sponge and running more or less parallel to its lateral 
margins, and the other transverse to this, directed at right angles to the face of the wall. 
In the Lithistida the general arrangement of the spicules recalls that of the 
Pachastrellidae, but the calthrops of the Pachastrellidae is replaced by its representative, 
the desma, and by the union of desmas a more or less rigid network results. 
The radiate type is that which prevails in the Choristida : it occurs throughout the 
Tetillidae and the genus Thenea, and in most of the Euastrosa and Sterrastrosa. In young 
sponges of this type the spicules lie in radial sheaves between the incurrent invagina- 
tions of the choanosome ; the rhabdomes of the triaenes extend from centre to circumference, 
as do the associated rhabdi, and the cladomes of the triaenes extend immediately beneath 
the external epithelium. 
As growth proceeds the spicules increase in length, but not rapidly enough to 
keep pace with the tracts of tissue in which they lie ; fresh spicules therefore make 
their appearance at the centrifugal ends of those first formed, and thus the spicule sheaf 
becomes elongated into a spicular fibre ; when the fibre remains short compared with the 
length of the spicules it will still be occasionally referred to a spicule sheaf. In the 
Tetillidae the radial sheaves or fibres are in some species crossed more or less transversely 
by loosely scattered oxeas. 
By the projection of the radial spicules beyond the surface of the sponge a general 
