SPONGES.-HALLMANN. 



kinds of megascleres, in very small number, are scattered 

 through the ground tissues and dermal membrane. Chelae are 

 fairly abundant in the dermal membrane, or at least in some 

 portions of it ; but are rare in the interior. 



Megascleres. 



(i.) The smooth (auxiliary) spicules show all gradations 

 in form between elongately-" headed" subtylostyli 

 and asymmetrical oxea ; 

 they are rarely less than 

 140 p, and usually between 

 160 and 170 fd in length, 

 whilst their maximum size 

 is about 200 X 3 p. 



i(ii.) The spined accessory spi- 

 cules are styli and oxea of 

 similar dimensions, the 

 latter occurring in relatively 

 small number. The spines 

 are small, and scattered 

 over the whole length of the 

 spicule. The maximum size 

 is 80 X 5 ji ; the length is 



rarely less than 50 ji and 

 seldom more than 70 y. 



Fig. 52a— IV. oxyphila. 

 a Chelae, b Acanthostyli. 



Microscleres.- — 



Isochelae of slightly variable shape, ranging in length from 

 20 to 26 p. As a rule, as seen in profile rheir form approaches 

 that of palmate isochelae, but the shafi is slightly curved and 

 makes a rather wide angle with the /ront palm ; a small pro- 

 portion bear a close resemblance to arcuate isochelae. Their 

 mode of development, however, indicates that their relation- 

 ship is rather with the arcuate type of chela. In their earliest 

 developmental stage they appear as a slender, slightly curved 

 rod ; the rudiments of the alae next make their appearance, ex- 

 tending along the shaft at either end for about one-third of its 

 length ; and finally the front palm is added. In the case of 

 quite an appreciable number of the chelae — which otherwise 

 attain to full proportions and indeed are usually somewhat 

 longer than the average — the palm remains relatively small 

 and the spicule then often bears a striking resemblance to in- 

 completely developed chelae of IVilsonella curvichela. The 

 normal form is that of the shortest of the three chelae shown in 

 the adjoining text figure, though the spicule is usually not so 

 robust. 



