THE SPONGES. 141 



Sjncuks. Megascleres. 1. Tylostyle, Fig. 8, a-c, Plate 18; smooth, 

 very slightly curved, with small head. Spicule tapers slightly toward tylote 

 end as well as toward pointed end, but in the smaller sizes the tapering 

 toward the tylote end is scarcely perceptible. Size ranges from 630 fi x 

 16 fji to 240 fi X S fx. The smaller sizes — 240 to 350 /a in length — pre- 

 dominate in the surface brushes and the vestibular membranes ; the larger, 

 in the radial tracts and the loose skeleton of the interior. 



Microsderes. 2. Sigmata, Fig. 9, a-c, Plate 18. Length ranges from 

 45 to 10 /x; common sizes from 36 to 12 /a in length. The sigmata are 

 abundant in the parenchyma and general dermal membrane ; only scantily 

 present in dermal membrane over the larger vestibular spaces. 



Skeletal Arrangement. 



lu the deeper parts of the sponge which are occupied by the He.xacti- 

 nellid skeleton, tylostyles are scattered separately and in slender short 

 tracts, without arrangement. In the superficial region numerous radial 

 or obliquely radial tracts extend toward the surface, there ending iu pro- 

 jecting brushes composed of diverging short tylostyles (Fig. 4, Plate 22; 

 Fig. 1, Plate 23). Spongin appears to be absent. At any rate, it was 

 not to be observed either in balsam or glycerine sections or teased prepa- 

 rations. 



While the surface in general is covered with the projecting brushes, 

 between which small subdermal cavities very commonly lie, in the dermal 

 membrane covering the vestibular spaces the surface skeleton has a different 

 character. The obliquely radial tracts which are adjacent to such a space 

 extend out into the covering membrane (Fig. 4, Plate 22, section ver- 

 tical to the surface ; Fig. 3, Plate 23, surface view of comparatively 

 large vestibular space with some of the surrounding area), thus coming 

 to occupy a tangential or nearly tangential position. In the case of the 

 smaller and medium-sized spaces the tangential tracts, as they pass from 

 the margin toward the middle of the vestibular membrane, preserve their 

 individuality (Fig. 3, Plate 23). In some cases, the whole tract occu- 

 pies a tangential position, the terminal spicules spreading out fan-wise in 

 the horizontal plane. In other cases, while the body of the tract lies 

 tangentially, the terminal spicules form a diverging bunch which points 

 obliquely upward much like tlie bunches of spicules found over the general 

 surface. Both conditions appear in Fig. 3, Plate 23. 



In the tracts of spicules which extend out into the membranes covering 



