THE SPONGES. 89 



but again much greater, as in Fig. 2, Plate 13. The margin is whitish 

 and tumid, projecting out as a thin membrane where the spicular fringe is 

 well developed ; the membrane including the base." of the fringe spicules. 

 The fringe itself is in most cases well developed along the upper mar- 

 gin, feebly developed or absent along the lower margin of the area. But 

 in a few areas it is well developed on both margins, and in a few is 

 absent from both margins. The areas are depressed, and the pore mem- 

 brane lining the area covers in a subdermal cavity which is continued into 

 several afferent canals. The flagellated chambers are eurypylous, and 

 measure about 50 fx in diameter. 



Me(/ascleres. 



1. Dichotriaene ; rhabdome, 3.5 mm. long with greatest thickness of 

 70 /x; protocladus, 180 /a x 50 /x; deuterocladus, 1.0 mm. long with basal 

 thickness of 35 /x. Smaller forms are abundant, down to such as have a 

 rhabdome, 2.2 mm. x 52 /x, protocladus, 125 /x long, deuterocladus, 440 /x 

 long. 



In most of the spicules, the rhabdome is thickest immediately below 

 the cladome, thence tapering to the point. In some spicules, the rhabdome 

 is slightly constricted immediately below the cladome, then expanding 

 and subsequently narrowing and tapering uniformly to the point, as in 

 the protriaene shown in Fig. 6, Plate 13. The greater part of the 

 rhabdome is quite slender, and somewhat curved. The protocladus is 

 inclined more or less upward, thus making an angle of 80° to 45° with 

 the rhabdome prolonged ; angle commonly near 70°. The deuterocladus 

 is straight, or more often slightly curved upward near the base, thence 

 about straight. 



^. Protriaene, Figs. 4, 6, Plate 13 ; rhabdome, 3-4 mm. long with a 

 greatest thickness of 50-60 /x; cladus, 400-700 /a long x 35-40 ^L thick at 

 the base. 



The rhabdome exhibits a dilated portion below the cladome. The cladi 

 make an angle of about 45° with the rhabdome, and are usually curved, 

 rarely nearly straight. The curvature of the cladi varies considerably in 



character (compare Figs. 4 and 6, Plate 13). The protriaenes are 



obviously modified dichotriaenes. Transitional forms to the dichotriaene 

 (Fig. 7, Plate 13) are rather more abundant than the perfect protriaene. 

 In these transitional forms the relative lengths of protocladus and deutero- 

 cladus are very variable. The protriaenes, perfect and imperfect, are 



12 



