PRINCIPAL GROUPS OF THE SPONGES. 



171 



nucleus, and may reach a foot or more in diameter, but is not 

 attached by a stem to the sea-bottom. In the nearly allied Ischad- 

 ites, the organism is conical, ovate, or pyriform in shape, with a 

 summit-aperture leading into a central cavity. In Receptaculites, on 

 the other hand, there is no distinct evidence that the cup-shaped 

 cavity was ever roofed over as it is in Ischadites. The internal 



Fig. 61. — Morphology ot Receptaculites. A, Outline of a perfect, basin-shaped specimen of 

 Receptaculitcs Xeptunf, viewed in profile, ot the natural size ; b, Part of the outer integument 

 of the same, enlarged two diameters, and so far weathered as to show the four transverse or 

 horizontal rays which underlie the summit-plates of the skeleton-spicules ; c, Side-view_ of a 

 fragment of the same, showing the skeletal spicules ; D, Vertical section of the same, magnified, 

 showing the skeletal spicules with their axial canals and expanded extremities. From the De- 

 vonian of Germany. (After Giimbel.) 



structure of these singular organisms will be best understood by refer- 

 ence to fig. 62, which is an ideal figure constructed by Mr Billings 

 to show the morphological characters of Receptaculites. It should 

 be noted, however, that this figure, though well representing the 

 structure of Receptaculites, gives an incorrect idea of the form of the 

 organism ; since the known species of this genus are cup-shaped 

 and open superiorly, as shown in fig. 60, a. The cup-shaped body 

 of Receptaculites is furnished with a thick wall composed of pillar- 

 like spicules arranged at right angles to the surface. These spicules 

 are shown in thin sections (fig. 61, d) to be furnished in their in- 

 terior with an axial canal, and they are expanded at their outer ends 



