42 THE SPONGES. 



arranged in large measure parallel to the free margin of the sponge. 

 Mingled with these are some of the other common, small parenchymalia, 

 intermediate between the diact and hexact conditions. The band projects 

 on the gastral surface, and while larger, is essentially similar to the slender 

 skeletal bands which elsewhere project on this surface. The marginal 

 stauractines lie on the dermal side of the band. 



In R. phoenix Schulze describes the margin of the sieve-plate area (1887, 

 p. 84, Plate XIII.) as surrounded by large dermal hexacts, the long spinous 

 distal rays of which project radially from the surface to a distance of about 

 5 mm. Ijima also finds enlarged dermal hexacts, which " lie crowded on 

 the cuff edge" (1901, p. 275), but the spicules are much smaller, the distal 

 ray measuring 1 to 1.25 mm. in length. Large dermalia of this character 

 are not found in the marginal region of R. dclicata, unless the single 

 spicule shown lying on the marginal band in Fig. 7, Plate 3, belongs in 

 this category. 



In the extreme marginal region of H. delicata many of the dermalia 

 have probably been lost, but groups remain here and there. Among these 

 are hexacts which do not differ from the common forms of dermalia 

 found in the upper part of the sponge. Several such are shown in Fig. 8, 

 Plate 3. In the upper left corner a fairly typical dermal hexact is 

 figured. Lower down lie other hexacts differing considerably in size and 

 detailed character. 



In some of the larger dermalia, near the margin, the distal rays are 

 reduced to conical bosses, and the proximal rays are but little longer. 

 A group of three such spicules is shown in Fig. 8, Plate 3 (to the 

 right). The tangential rays here, as in the other enlarged dermalia, are 

 subequal or strongly unequal. The condition of these spicules is not far 

 from that of the marginal stauractines, and it seems probable that it is from 

 them that the stauractines are recruited as the sponge increases in size. 

 That is, the condition of the marginal region of this specimen suggests 

 that an enlarged dermal hexact, in which one of the tangential rays is 

 especially elongated, is from time to time pushed to the edge, the tangen- 

 tial ray in question becoming the projecting superior ray of the stauract. 



