PRINCIPAL GROUPS OF THE SPONGES. 



i/7 



acters of which are in some respects anomalous. In this curious 

 genus (fig. 6S, c and d) the sponge-body is elliptical, greatly com- 

 pressed, and free, without any traces of a canal-system. The basal 



Fig. 68. — a, Side-view of a specimen of Asirceospongia meniscus, of the natural size, Sil- 

 urian : b, Spicules of the same, enlarged (after Roemer) : c, A split specimen of Atnphi- 

 spongia oblonga, of the natural size, Silurian ; d, Part of the upper portion of the same, 

 enlarged. (Original.) 



portion of the sponge consists of large conical spicules, arranged 



with their pointed lower ends converging towards the middle line ; 



while the upper portion consists of :i slender four- and five-rayed 



spicules with the rays at right angles to each 



other " (Hinde). The spicules are not fused 



with one another, and their arrangement is 



peculiar. In spite of its aberrant character, 



however, it would seem that Amphispongia 



should be regarded as belonging to the 



Hexactinellidce. 



Order 7. Octactixellid.e. — This order 

 is represented only by the single genus As- 

 trceospo?igia, and is characterised by the pre- 

 sence of skeleton-spicules which are normally 

 eight-rayed, six of the rays radiating at equal 

 angles from a central point (fig. 68, b), while 

 the other two rays form a vertical axis. The 

 spicules are not fused with one another, and 

 the vertical rays are often obsolete or wholly 

 wanting. The sponges which constitute the 

 genus Astrceospongia (fig. 68, a) are discoid 

 or basin-shaped in form, without any stem of 

 attachment, and without any definite canal- 

 system. The spicules have the form above 

 described, and are now usually found in the 



condition of calcite. According to the views of Dr Hinde, however, 

 the present calcareous condition of the spicules is the result of 

 secondary changes or replacement, and the skeleton was originally 



vol. 1. M 



Fig. 69.— Skeleton-spicule 

 of Astcractinclla expansa, 

 one of the Heteractinellid 

 Sponges, from the Carbon- 

 iferous rocks, enlarged ten 

 diameters. (After Hinde.) 



