STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 5 



straight and cylindrical, with a pore-sieve opening into it at 

 cue end and a vent at the other. 



Such, then, was the state of our knowledge of Grantia 

 labyrinthica, one of the most remarkable of all the calca- 

 reous sponges, when I commenced my investigations. How 

 far my own observations agree or disagree with Mr. Carter's 

 descriptions will appear subsequently. 



{b) External Characters. 



Grantia labyrinthica is very large for a calcareous 

 sponge, well-grown specimens being about three inches in 

 height and a little more in breadth, so that it is probably the 

 largest of all the Sycons. Hence it is peculiarly well adapted 

 for anatomical investigation. 



A better idea of the external appearance will perhaps be 

 gathered from an examination of fig. 4 than from any descrip- 

 tion which I can give. It will be seen that the adult sponge 

 consists essentially of a thin- walled cup or basin, with a widely 

 open mouth showing no signs of constriction, and thus differ- 

 ing markedly from the oscula of all other known Sycons. 

 The wall of the cup, however, which, as I shall show later on, 

 is in the young sponge simple and not folded, in the adult 

 becomes greatly convoluted and folded upon itself, without, 

 however, ever losing its character of a single continuous 

 lamella. Thus, while in most Sycou sponges the circumfer- 

 ential growth of the tube or cup after a time diminishes as the 

 sponge grows older, giving rise to a more or less constricted 

 osculum, in Grantia labyrinthica precisely the reverse 

 takes place, and the circumferential growth so far outstrips 

 the vertical growth that the sponge wall becomes thrown into 

 numerous deep folds, while the osculum becomes enormously 

 wide and bounded by a deeply sinuous margin. The cup 

 thus formed is attached by the middle of its lower surface to 

 a stout cylindrical stalk, which is also a subsequent develop- 

 ment not present in the very young sponge. At its lower end 

 the stalk is fixed to the rock or other body upon which the 

 free swimming embryo may have chanced to come to rest. 



