472 ASTROSCLERA WILLEYANA, THE TYPE OF A NEW FAMILY OF SPONGES. 



In von Koch's account of the development of the skeleton in Asteroides caly- 

 cularis^ its elements at their first appearance are described as spheroidal bodies, which 

 become polyhedral by apposition and possess a radiate structure (PL 20, Fig. 7). They 

 have the appearance of being partially divided into two pieces. Von Koch regarded 

 these bodies, as well as the later formed skeleton, as excretions, formed by the calycoblast 

 {epiblast) cells of the coral. 



The precise relation of the calycoblasts of corals to the skeleton which they form is 

 however still uncertain. The question is whether the skeleton is, as von Koch and other 

 authors have supposed, secreted by the layer of cells in contact with it, or built up of 

 cells of this layer which themselves become calcified. Miss Ogilvie urges strong reasons 

 in favour of the latter view. 



In either case it is not clear what the relation of the spherules described by 

 von Koch to the fully-formed skeleton may be, for in this such spheroidal elements do 

 not appear to be present. 



I mention these spherules here because they are skeletal elements which corre- 

 spond in composition (aragonite), and to more extent in structure, with those of 

 Astrosclera, but not as evidence of any close affinity of the latter with the Coelen- 

 terates. 



But, admitting that Astrosclera is a sponge, there are many features which 

 separate it from the living members of this group. Among these may be mentioned : — 



(a) The shape of the skeletal elements. They are polyhedra which begin in a 

 spherical and may pass through a spheraster stage. 



(6) Their union to form a rigid skeleton, excluding the soft parts. In Petrostroma 

 Doderlein, the representative of the Lithonina'-, the supporting skeleton is formed of 

 fused spicules, but these are modified quadriradiates. We are at present without 

 information on the soft tissues of this form. 



(c) The mode of growth, by the addition of new skeletal elements at the upper 

 surface, and without interstitial growth. In this feature also it probably agrees with 

 Petrostroma. 



(d) The limitation of the pores to the upper surface. 



Tentorium (Vosmaer), a siliceous sponge classed with the Polymastidae, presents a 

 similar limitation. 



(e) The absence of a central atrial space. 



(/) The small size (18 by 11/i) of the ciliated chambers. The smallest size given 

 by Haeckel for the ciliated chambers of the Leucones (in which gi-oup they are smaDer 

 than those of other Calcarea) is 60 by 40/x. Among the Non-calcarea, 42/a is the 

 smallest diameter that I have found given for the ciliated chambers. 



1 "Ueber d. Entwickelung d. Kalkskeletes von Asteroides calycidaris u. dessen morphologische Bedeutimg." 

 Mitth. aiis d. Zool. Stat, zu Neapel, Vol. 3, 1882, p. 284. 



' Doderlein, L., "Ueber die Lithonina, eine neue Gruppe von Kalkschwammen. " Zool. Jahrbiicher, Syst. 

 Abth. X. (1898), p. 1.5. 



