5;J6 MK. A. T. JENNINGS ON A 



the typical A. Millari lies iu tlie absence of the smooth microxeas 

 (subskeleton spicules, Carter) so abundant in that specimen. 



Through the kindness of Prof. Martin Duncan, E.E.S., I have 

 been able to examine preparations of the original specimen, and 

 the perfect series of gradations from the smooth acerate microxea 

 to the " sceptrella " type is very striking. The simple forms pass 

 into those with scattered papillae, and these again into the typical 

 microsclere with its two whorls. It may be noticed also that 

 these " subskeleton-spicules " are more markedly diactinal than 

 the skeleton-spicules ; they are bent at varying angles, and, as 

 Prof Duncan pointed out to me, in some cases a well-marked 

 projection at the angle looks like a rudimentary third ray. 



Inclusion of Foreign Spicules. — The incorporation of a hetero- 

 geneous aggregate of spicules, sand-grains, foraminifera, &c. iu 

 the fibrous network of the skeleton is a familiar feature in certain 

 sponges. Equally well known is the chance occurrence of a 

 foreign spicule embedded in the soft tissues. The phenomenon 

 of the presence of a number of similar spicules of one kind in the 

 tissues of a sponge which has no general habit of accumulating 

 foreign bodies is of a different nature, and important on account 

 of the errors it might in particular cases occasion. 



In 1880 Mr. S. 0. Eidley* brought before the notice of the 

 Society two cases of monaxonid sponges which had included 

 spicules belonging to other genera; but, so far as I know, similar 

 instances have not been recorded in the boring-sponges, which 

 are indeed the last group in which such an occurrence would 

 be expected. 



Among the spicules first examined from the cavities excavated 

 in the shell, there occurred a considerable number of rounded 

 triangular disks, evidently derived from the dermal layer of some 

 species of Discodermia. The explanation at first seemed to be 

 that they had been accidentally washed into the cavities after the 

 death of the sponge ; but the absence of other foreign bodies, and 

 the fact that they occur in different parts embedded in the dried 

 sarcode together with the proper spicules, rendered that expla- 

 nation untenable. Moreover, I have since found them in another 

 specimen from Christiania, and, more important still, a group of 

 the same spicules occurs in a preparation of the original specimen 

 of Alectona Millari, kindly lent me by Dr. Hinde. 



* Journ. Liun. Soc, Zool. vol. xv. 1880, p. 149. 



