1890-91-] The Structure and Life-History of a sponge. 433 



ing. A good example is furnished by the well-known British 

 sponge, Grantia compressa, foimd on so many parts of our 

 coast, and named by Professor Fleming in honour of Professor 

 Piobert Grant. The spicules in this sponge are calcareous, and 

 are of two forms, clavate and tri-radiate, the latter predomi- 

 nating. The whole of the sj)onge-body is crowded with these 

 spicules, as may be seen in the section shown under the micro- 

 scope. Another example is the " crvmib- of -bread sponge" 

 {Hulicliondria or AviorjjJiina imnicca), common in the rock- 

 pools of both shores of the Firth of Forth, and many other 

 parts of the British coast. The needle-shaped silicious spicules 

 of this sponge are also so numerous as to form nearly its 

 entire mass.'^ But it is not only amongst the sponges that 

 these spicular elements are found, for they are present as well 

 among the related forms of the Ccelenterata and Echinoder- 

 mata, as in Gorgonia, Plexaura, Alcyonia, and Synapta. The 

 calcareous spicules or sclerites imbedded in the soft parts of 

 these animals also assume beautiful and varied forms, and are 

 often of brilliant colouring, forming lovely objects when seen 

 under the microscope. The bihamate or double-hooked shape 

 of spicule is found in several sponges, and reappears again in 

 the tube-feet of the echini, as mentioned last session in my 

 paper on the Sea-urchins.' Thus, as has been well remarked, 

 " we find in the spicula only, a series of links in the chain of 

 animal development, immediately connecting the Spongiadie 

 with the higher tribes of animals." 



The water-system of the sponges, which may now be 

 noticed, forms a very interesting study. Professor Grant, in 

 1825, was the first to witness the beautiful phenomenon of 



1 FzVfe Note on "Sea-fyke," at the end of this paper, p. 437. The late P. 

 H. Gosse, in one of his extremely interesting popular works on natural history, 

 describes the spicules in the living sponge in such a simple yet apposite manner, 

 that I am tempted to quote the passage here. He says: "If you have ever 

 shaken up a box of dressing-pins, and have then endeavoured to take one out, 

 j'ou know how bj' their mere interlacement they adhere together in a mass, so 

 that by taking hold of one you may lift a bristling group of scores. Somewhat 

 on the same principle are the calcareous and silicious pins (sjjiada) of a sponge 

 held together by nnitual interlacement. Yet their cohesion is aided by the ten- 

 acity of the living sarcode which invests them ; for I have found that specimens 

 of Grantia, . . . when long macerated in water, so that the sarcode is dissolved, 

 have very slight power of cohesion among their spicula." — 'Evenings at the 

 Microscope,' by Philip Henry Gosse, F.R.S., pp. 385, 386 (ed. ISSi: S.P.C.K.) 



" " On the Echinoidea or Sea-urchins," ante, p. 351. 



