Ixxviii 
THE VOYAGE OF H.M.S. CHALLENGER. 
present they should diverge from each other at three equal angles, or two at least should 
be equal, hence the preponderance of regular and sagittal microtriods amongst the triac- 
tinose asters ; if four actines are present then they should make angles of 90° with each 
other or be directed as normals to the faces of a tetrahedron, and such we find to be the 
case ; if six are present they might be variously arranged, but we should expect to find 
them frequently directed along three rectangular axes, as we so generally do. In certain 
cases, as amongst more or less closely adjacent spherical flagellated chambers, the tensions 
due to the presence of these spherical surfaces will be so directed as to distinctly favour 
the calthrops form, and thus in Placina, which possesses but little mesoderm, and in which 
consequently the chambers are crowded together, the calthrops is the prevailing form of 
microsclere. The Hexactinellids do not fall within the province of this Eeport and 
therefore will not be considered. While the asters of the Theneidse clearly indicate the 
supersession of the influence of intracellular tensions by those of tissues or of the organism, 
there are asters in other families of which this cannot be said ; thus the minute chiasters 
of the Stellettidse, and other minute forms of both the Euastrosa and the Sterrastrosa, are 
only slightly affected by extracellular tensions, but as these pass into larger varieties 
the same influences are brought to bear as in the Theneidae, and large asters with 
deflnitely directed actines as in that family result. The aster which attains the largest 
size of all microscleres is the sterraeter ; this commences in many of the Geodiidse as a 
perfectly spherical sclere, with an infinite number of equal and similar actines of hair- 
like fineness, obviously developed under the action of radial tensions in a spheroidal 
scleroblast, which persists and can be observed up to the almost completed growth of 
the sclere ; the circum-nuclear protoplasm is in this case inactive so far as the secretion 
of silica is concerned, and thus the position of the nucleus is always indicated by a 
hilum. The uniform growth of the sterraster along an infinite number of radii is to 
some extent perhaps connected with the abundant mesoderm which characterises the 
sponges in which it is produced, and to the fact that it completes its growth within 
the choanosome ; but even in this spicule the influence of extracellular tensions is made 
manifest, for it is only the smaller forms which are almost perfectly spherical, in the 
larger the sclere is invariably ellipsoidal. Again in one group of the Sterrastrosa, the 
Erylina, the influence of the extrinsic tensions produces still further changes in the 
general form of the sclere, the actines in the equatorial plane being immensely over- 
developed compared with those in other directions, and thus a lenticular sterraster is 
produced ; a still further change in form arises by the disproportionate growth of the 
actines along one of the equatorial axes, by which the lens becomes converted into a 
somewhat fusiform lozenge, which thus resembles a rhabdus in general form, while it 
retains the sterrastral structure. That this transformation is connected with lines of 
stress in the organism is suggested by the fact that the euasters are correspondingly 
modified, most of them suffering a reduction in the number of the actines, by which 
