124 



Transactions of the Society. 



upwards and towards the axis of the cup, and frec[uently prolonged 

 at the sides into a little gutter, as in Fig. 1 . 



Fig. 1. 



Fig. 2. 



0, osculo ; c, excurrent canal ; g, groove. 



These holes are the mouths of the excurrent canals, which 

 descend into the walls of the sponge, passing especially into the 

 ridges of the outer surface, where, after branching once or oftener, 

 they terminate, either in small round openings on the surface, or 

 by losing themselves in the coarse meshes of the skeleton. 



In a similar manner the grooves or gullies of the outer surface 

 are prolonged into tubes which tend towards the inner surface of 

 the cup, ramifying in their course till they open into the excurrent 

 canals, or lose themselves in the large meshes of the skeletal 

 network. 



Thus the only connection between the excurrent canals which 

 open on the inside of the cup, and the deep gullies of the exterior, 

 is by means of very minute intervening canals, or through the 

 large meshes of the skeleton. 



The ridging and grooving of the exterior, combined with 

 the excavation of the ridges by the excurrent canals, produce a 

 folding of the sponge wall, very similar 

 to that which occurs in the Ventricu- 

 lites and other fossil sponges. In both 

 cases the folding serves to give great 

 strength to the sponge wall, and a large 

 inhalent surface at a great economy of 

 space. 



Section across the wall of The whole arrangement reminds one 

 D. pumceus (| natural size) ; ^^so of what is Seen on a smaller scale in 



r r, exterior ridges ; 1 1, inter- tt ?• 777- i 1 -i 



voning furrows ; e e, excurrent Haiisarca lotmlaris, where likewise we 

 canals occupying interior of have, according to the beautiful sec- 

 "^g«- tions of F. Eilhard Schultze,* incurrent 



canals opening externally and branching within into minute canals, 

 which again gather together to form the large excurrent canals 

 that open on the interior of the sponge. Here, however, having a 

 fresh specimen before us complete in all its parts, we can see the 

 ampullaceous sacs on the ultimate ramifications of the incurrent 

 canals, and so understand clearly the mechanism by which water is 



* ' Zeitschriit f. wiss. Zool.,' Bd. xxviii. Taf. I. fig. 8 ; Taf. II. fig. 15 ; 

 Taf. III. fig. 10. 



