Mr. H. J. Carter on the Hexactinellidse. 353 



whole structure together; for, mdeed, if divested of this filigreed 

 development, as is the case with the sui-face-spicules of Dacty- 

 locaJyx Boiverhanhii^ Johnson (which are simply trifid-headed 

 shafts with the shaft projecting internally and the trifid heads 

 horizontally), they would be reduced to the state of similar 

 spicules in the Geodinidffi and in Pachastrella (that is, held 

 together only by the amorphous sarcode of the dermis), and 

 would thus form a group in my fifth division. 



We shall see by-and-by that, while the glassy skeleton of 

 the Hexactinellidse is formed by a vitrification of the sarcode 

 or homy fibre, that of the Lithistid^ is fonned by a vitrified 

 extension of the spicule. 



Assuming this, I shall for the present give lists respec- 

 tively of the Hexactinellidaj and Lithistidaj which have already 

 been described, grouping them, according to the best of my 

 judgment, as their characters seem to indicate, and following 

 each by a short commentary. After which I shall take the 

 branched tube-net sponges (that is, the Aphrocallistida^, in 

 which we must include Farrea and Aulodictyon) for the special 

 subject of this paper, returning to the other groups for more 

 detailed description at a future time when it may be more con- 

 venient. I shall also, for the present, use the tenns "sex- 

 radiate" for the Hexactinellidse, and " quadriradiate" for the 

 Lithistid^e. 



As the spicule of the Hexactinellidee and-Lithistidaj is, like 

 that of all other sponges, developed from a single point or 

 minute cell, which, by linear extension, becomes the central 

 canal on which the vitreous layers of the spicule are sub- 

 sequently built, we find that in the Hexactinellidai the spicules 

 are produced by a more or less uniform radiation from the 

 central cell immediately ,• while in the Lithistidaj the spicules 

 are formed by a more or less uniform triradiation subsequently^ 

 or from one extremity of the primary cell after it has un- 

 dergone linear extension, — that is, that this half of the shaft 

 is transformed into three arms, and that, too, probably com- 

 mencing a little heyond the central cell, as an inflation of the 

 shaft, often just below the head, leaving a kind of neck or more 

 constricted portion, would seem to indicate. (See especially the 

 large trifid spicules of the Geodinidaj &c.) 



With reference to the Hexactinellida^, it is also necessaiy to 

 premise that there are here, as in most of the Spongiadie, large 

 spicules which are only concerned in the formation of the sup- 

 porting structure or skeleton, which we shall temi " skeleton- 

 .s})icules," and minute spicules which belong entirely to the 

 sarcode, which will be termed " flcsh-s})icules" — also that the 

 latter here often afford a valuable character in conjunction wit]i 



