134 CEYLON PEARL OYSTER REPORT. 



In dealing with the Astromonaxonellida I have already had occasion to point out 

 that no reliable instance of the occurrence of true astrose microscleres in conjunction 

 with sigmatose forms has ever been recorded, a fact which argues very strongly in 

 favour of the primary cleavage of the Monaxonellida into two great groups corre- 

 sponding to the tetractinellid sub-orders Astrophora and Sigmatophora. Since that 

 part of my report was completed, however, and sent to England, Topsent's latest work 

 on the ' Sponges of the Azores ' (62), has been received here. In this work the author 

 describes two apparent Sigmatomonaxonellida in which asters were met with, viz., 

 Yvesia alecto and Leptosastra constellate/, but in neither of these are other microscleres 

 present. 



In Yvesia alecto the asters are, as Topsent himself shows, simply transformed 

 megascleres (spined oxea) with the spines greatly developed, so that they are clearly 

 of secondary origin. It may be pointed out, further, that an analogous transforma- 

 tion takes place in the echinating megascleres of the genus Cyamon * and, probably, 

 also in the genus Trihentrion, giving rise to spicules which simulate true asters. It 

 appears, therefore, that in certain Sigmatomonaxonellida astrose spicules have arisen 

 secondarily by transformation of spined megascleres. It is obvious, however, that 

 these cases do not affect the primary division of the Monaxonellida here adopted. 



The case of Leptosastra coustellata offers a more serious difficulty, for here there is 

 no indication that the asters, which form a superficial crust, are other than true 

 astrose microscleres. It is, however, a suggestive fact that the sponge in which they 

 occur is an Ectyonine with spined styli. Two explanations appear to me possible : 

 (1.) The asters may not belong to the same sponge as the megascleres. Considering 

 the well-known and frequent accidental admixture of the spicules of different species, 

 caused either by the sponges growing over one another or by the taking in of foreign 

 spicules in the same way that grains of sand are taken in, and especially in view of 

 the fact that only a single specimen of the sponge has been obtained ; although 

 Topsent has assured himself to the contrary, I venture to think that we may be here 

 dealing with a composite spiculation. (2.) It is not impossible that the asters, if 

 proper to the sponge, may be derived secondarily from the spined styli, in much the 

 same way as in the genus Cyamon. Their position at the surface of the sponge, 

 however, and their apparently normal form, are opposed to this view. However, 

 until we have further evidence before us, it is quite unnecessary to allow this isolated 

 case of a single specimen to make us alter our views on the classification of the 

 Monaxonellida. 



Assuming;- then that the Sip'matomonaxonellida are derived from the tetractinellid 

 Sigmatophora, the question arises where are we to seek for the point of contact 

 between the two groups ? The answer to this question is easily given, for in the 

 genus Gellius we have a near approach to the massive species of Tetilla, such as 

 T. limicola. The replacement of the tetractinellid megascleres by oxea in a massive 



* See later xmdeTjCyamon. 



