SPONGES 



ax.t. 



developed on the primary ray, and, on the other hand, a minute 

 continuation of the axial thread may often be found indicating a 

 ray which has been completely lost. A beautiful instance of the 

 latter kind is seen in the diactines which have 

 the two rays placed in the same straight line 

 (secondary monaxons). In some instances the 

 four undeveloped rays are indicated by four 

 knobs, containing as many axial canals, which 

 form a minute axial cross at the morphological 

 centre of the spicule (Fig. 83, A). In other 

 cases the four knobs are further reduced to a 

 slight swelling, or have disappeared altogether 

 (Fig. 83, B, C), the minute axial cross remaining, 

 however, to indicate the aborted rays. Finally, 

 even the axial cross may disappear, leaving no 



trace of the missing rays. 



A. B. 



Flo 83. The ro t tuft with which many Hexactinellids 



Three stages in the re- are provided is composed of long thread-like spicules, 



duction of a hexactine which in Hyalonema may be two feet or more in 



to the monaxon condi- , ,, , . .,, ., 



tion. in A four nidi- length, and are furnished with recurved, anchor- 



srtfdVs^anVnXi like hooks at their distal extremities. Some of 

 in B there is only a these rooting spicules bear at their termination four 

 place ; ^ (Tfhey ha^e hooks, placed at right angles to each other, and to 



disappeared altogether, the shaft, and containing prolongations of the axial 



In all three the lost rays ... . 1 . 



are indicated by the canal ; the spicule is therefore pentactmal, with one 



? Vefy ^^ devel P ed - In others the anchor- 

 ing hooks are numerous and arranged according 

 to various types of symmetry ; they contain no axial canal, and are 

 therefore of secondary origin, but at some point in the shaft of the 

 spicule a minute axial cross can usually be found, proving it to be a 

 much elongated diactine. In a similar way the scapulae (Fig. 82, c) are 

 seen to be diactinal in their nature, the axial thread not being con- 

 tinued into the terminal branches. 



Arrangement of the Spicules in the Skeleton. According to their 

 position in the sponge body the spicules of Hexactinellids may be 

 divided into several categories, corresponding to the regions of the 

 body which it is their function to support or protect. 



(1) Prostalia. Defensive spicules, usually diactinal monaxons, 

 which project over the surface of the body, only found in Lyssacina. 

 A special differentiation of such spicules may form a protecting 

 fringe round the osculum, or an anchoring root tuft at the base 

 (prostalia marginalia et basalia). Those scattered over the general 

 surface of the body are termed pleuralia. 



(2) Dermalia. Spicules supporting the dermal membrane ; 

 usually hexactinal or pentactinal, with four similar rays lying em- 

 bedded in the membrane. They are distinguished as autodermalia, 



