SPONGES. 385 



form beautiful microscopic objects for the binocular ; they 

 must be mounted dry, and viewed either with the dark 

 ground illuminator, or by condensed light. 



8PONGIAD.E. SPONGES. 



The term Porifera, or Canal-bearing Zoophyte, was 

 applied by Professor Grant to designate the remarkable 

 class of organized beings known as sponges, which are met 

 with in every sea, growing in great abundance on the 

 surface of rocks. 



Ellis, in the course of his investigations, was astounded 

 by discovering that sponges possessed a system of pores 

 and vessels, through which sea-water passed, with all the 

 appearance of the regular circulation of fluids in animal 

 bodies, and for the seeming purpose of conveying ani- 

 malcules to the animals for food. 



The description given of sponges by Dr. Johnston is, 

 that they are " organized bodies growing in a variety of 

 forms, permanently rooted, unmoving and irritable, fleshy, 

 fibro-reticular, or irregularly cellular ; elastic and bibulous, 

 composed of a tibro-corneous axis or skeleton, often inter- 

 woven with siliceous or calcareous spicula, and containing 

 an organic gelatine in the interstices and interior canals ; 

 and are reproduced by gelatinous granules called gem- 

 mules, which are generated in the interior, but in no 

 special organ. All are aquatic, and with few exceptions 

 marine." l Our author continues : " Mr. John Hogg, in a 

 letter to me dated June 25, states that the green colour 

 of the fresh-water sponge (Spongilla fluviatilis) depends 

 upon the action of light, as he has proved by experi- 

 ments which showed that pale-coloured specimens became 

 green when they were exposed for a few days to the 

 light and full rays of the sun ; while, on the contrary, 

 green specimens were blanched by being made to grow 

 in darkness or shade." 



The living sponge, when highly magnified, exhibits a 

 reticulated structure, permeated by pores, which united 

 into cells or tubes, ramify through the mass in every 

 direction, and terminate in larger openings. In most 



(1) See Dr. Johnston's History of British Sponges, atd Mr. Bowerbank's revi- 

 sion of the class, in the publications of the Kay Socieay. 

 C C 



