214 A MONOGRAPH OF THE 



short extremity ; but when they begin to advance beyond 

 the surface of the shell, their extremity becomes flat and 

 slightly dilated, assumes a villous appearance, with open 

 fissures, radiating from the centre to the margin of the 

 papilla, and at length a minute circular opening is perceived 

 in the centre of the villous surface. The papilla advances 

 from the shell, and its central opening enlarges in propor- 

 tion to the healthy state of the Zoophyte, and the purity 

 and stillness of the water ; its flat, downy, radiated surface 

 gradually diminishes by the widening of the central opening, 

 till only thin margins are left around the orifice and the 

 current is again seen to play briskly from it." These 

 observations of Dr. Grant I had frequent opportunities of 

 verifying at Tenby, and I can therefore speak with con- 

 fidence as to their accuracy. In addition to the experi- 

 mental touches with a needle, I tried the effect of the 

 application, very delicately, of small drops of nitric and 

 hydrochloric acids, but these strongly stimulating fluids did 

 not cause the slightest contraction of the protruded portion 

 of the sponge. 



The same distinguished naturalist also states, that 

 " during the months of March and April, when his obser- 

 vations were made, numerous small yellow ova were seen 

 in the vicinity of the canals, agreeing much in their form, 

 colour, size, and mode of distribution, with those of the 

 Spongia papillaris and S. panicea, which were then nearly 

 in the same stage of advancement." 



Dr. Johnston, in his ' History of British Sponges,' p. 125, 

 in his description of the specific characters, says that " the 

 circular orifices are often filled with a mammillated plug ;" 

 but it is not clear from which of the two varieties he has 

 described of his Halichondria celata he has derived this 

 character. I have myself found such a mammillated plug 

 as he describes, in the surface orifices of the perforated 

 stones at Tenby, but I have satisfied myself that it was no 

 part of the sponge. The true oscular portion of the sponge 

 projected from the orifices of the tubular perforations 

 which it inhabits abound with the proper spicula of the 

 skeleton, and no appearance of mammillae can be detected 



