iiS 



SPONGES 



^ 



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

 continuation of the axial thread may often be found indicating a 

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

 latter kind is seen in the diactines which have 

 ,.',.. tlie 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, whicli 

 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 miinite axial cross remaining, 

 however, to indicate the aborted rays. Finally, 

 even the axial cross may disappear, leaving no 

 trace of the missing rays. 



W 



i 



A. 



Pj(j 53 The root tuft with wliich many Hexactinellids 



Three sta-'es in the re- ^^^ provided is Composed of long thread-like spicules, 



auction of a hexactine which in llmlonema niav be two feet or more in 



to the nionaxon condi- , , , e ■ ^ t ■ ^ i i 



tion. In .-1 four nidi- length, and are lunnshed with recurved, anchor- 

 nientary rays are repre 

 sented by suiaU knobs 

 ill li there is only 8 



ilisappeared altogether 

 In all three the lost rays 

 are indicated by the 

 minute axial cross in the 

 axial filament (n J. 0- 



like liooks at their distal extremities. Some of 

 these rooting spicules bear at their termination four 

 pi'ace ; ^n^ I'They h^ve hooks, placed at right angles to each other, and to 

 the shaft, and containing prolongations of the axial 

 canal ; the spicule is therefore pentactinal, with one 

 ray very greatly developed. 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 scopulac (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 jiroject over the surface of the body, only found in Lj/ssacina. 

 X s})eci:il diHerentiation of such spicules may form a ])rotecting 

 fringe rouml the osculuni, or an anchoring root tuft at the base 

 {prostalia marginalia ct hasalin). 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 ra3's lying em- 

 bedded in the membrane. They are distinguished as autodermalia. 



