190 



DlCTYOSPONGID^. 



FiatTRE 3S. Physospongta Dawaoni, A group of an- 

 Chora and a cleme. x60. (J. U. C.) 



])arts by the intersection of the horizontal with the secondary vertical l)ands. 

 Two of the quadrilles thus formed are elevated into regular convex pouches or 

 nodes, the other two are concave, and it seems probable that the curvature of 



the concavity was originally equal to that 

 of the node. These elevations and depres- 

 sions alternate in position and thus each 

 node is surrounded by four depressions 

 and each depression by four nodes. The 

 general surface effect produced by this 

 arrangement is that of a series of double 

 vertical rows of nodes separated by the 

 broad vertical spicular bands. The num- 

 ber of the primary vertical bands is usually 

 twelve, though some specimens seem to 

 have had not more than ten. 



Growth. The younger parts of the 

 specimens usually show a diminution in 

 the size and in the development of the surface nodes and depressions, and 

 when the actual basal parts of the cup become known they probably will be 

 found comparatively free of the usual surface conformation. Near the aper- 

 tural margin also the nodes become less distinct, that is, less elevated, though 

 often of greater area. The duplication of the nodes and depressions as an 

 accompaniment of growth in the expansion of the cup, such as occurs fre- 

 quently in P. CoUetti, is very rarely observed in this species. 



Sheleton. In the remarks already given upon the structure of the skele- 

 ton of the thin-fleshed Dictyosponges, it has been observed that the tuft of 

 basal spicules or basalia is but a continuation and union of the long vertical 

 spicular bundles which, with the horizontal bands, produce the characteristic 

 reticulation of the surface. Further evidence from analogy with living 

 hexactinellids, as well as from the facts themselves, is that these lateralia or 

 upward extensions of the basalia lay withm the sponge and probably bounded 

 the surface of the great gastral cavity. The flesh- wall of the sponge, however, 

 was so exceedingly tenuous that in the best preserved of the Dictyospoxgid^ 

 it is usually extremely difficult to distinguish the gastral from the dermal 

 surface as far as shown by differences of structure. 



In Physospangia Dawsoni the innermost or true gastral surface of the 

 principal lateralia bears only long cylindrical rods, varying notably in size. 

 These may frequently be followed for the length of two or three quadrules 



