318 



NATURAL HISTORY. 



other species, the "yellow" and "hard-head" Sponges of the American shores, resembling E. 

 zimocca ; and the "wool" Sponge, which appears to be one or perhaps two species of the Hippo- 

 spongia, //. yossypiua, and H. mectndriformis, the " velvet " Sponge. 



GENERAL CHARACTERS OF THE SPOXGIJE. 



In form and size Sponges vary greatly : some are no larger than a pin's head, others as 

 much as four feet in height and breadth, while some attain a length of over six feet. In form 

 they are massive ; incrusting, sessile, or stalked ; globular, branched, tree-like, with the branches 

 free or united laterally into a network; lamellar, irregularly or fan-shaped; tubular, vasiform, or 

 labyrinthic, many of the forms presenting a close parallelism to those of 

 Corals. In some the form is constant and characteristic, as in the fairy-like 

 Venus-basket (Euplectella, Fig. 9, A) ; the glass-rope Sponge with its cylindrical 

 body (Hyalonema, Fig. 9, B) ; the open Flower-basket Sponge (Dactyloci >!</.>; 

 Fig. 9, E) ; or the great Neptune's Drinking Cup (Poterion, Fig. 9,c iN ); but usually 

 it is very variable ; and since the same species may assume different 

 forms, and the same form be common to different species, external shape 

 is of very slight value in classification. The different forms can be derived 

 from each other in many cases by quite easy gradations. Thus from a massive 

 spreading Sponge may grow up linger-like extensions, and these, by branching, 

 give rise to a tree-like form. By the subsequent union of the branches a 

 net-like or clathrous stock results ; or the finger-like elevations may widen 

 into a lamella which, broadening as it grows, becomes fan-shaped ; growing 

 more rapidly on one face than the other, the fan becomes curved, and as 

 the curvature increases the approximated edges at length touch and join 

 together, producing a cup-like or vase-like form, the origin of which remains 

 clearly indicated by a hole near the base, where the sides of the fan failed 

 to reach, and still remain apart. 



The mass which we speak of as the Sponge may consist of a single indi- 

 ASCETTA visual or several, just as a Coral may be single or compound; but it is not 

 so easy in the case of the Sponge to determine what constitutes an 

 individual. Usually the osculum is taken as the characteristic mark of a 

 "person," but in some Sponges the osculum is absent (lipostomism) and the 

 excurrent canal opens by the pores. In this case the excurrent canal must be regarded as indicating the 

 individual, but again even this may disappear (lipogastrism), and then the question of individuality 

 becomes as puzzling as it would be in a Coral which had lost all its polypes and consisted only 

 of ccenosarc ; in this case \ve must regard as an individual the whole Sponge mass. The colours 

 of Sponges, which are very various, are usually due to the presence of pigment granules em- 

 bedded either in the endosarc of the flagellated cells, or in the mesodermic cells, usually of the 

 skin only, but sometimes of the whole body. The various tints range through the whole octave 

 of colour, the commonest perhaps being various shades of yellow and brown ; grass-green and 

 orange-red are frequent; rose red, faint lilac, deep carmine, sky-blue, indigo, black, are also not 

 rarely met with, as well as all the colours of flowers and of the leaf from the bud to the fall. 

 Sometimes the colour of the same species differs in different localities, as in Ascetta clathrus, 

 which, though usually grey, is sometimes sulphur-yellow or vermilion-reel. Many Sponges are 

 white as snow, and, for the same reason, their minute colourless transparent spicules scatter the 

 incident rays of light, just as the tangled crystals of a snow-flake do. Occasionally the colour of 

 the Sponge is accidental, as when it depends on that of ingested food particles, or of parasitic alga?. 

 Those pigments which belong to the chlorophyll group no doubt play the same part here as they 

 do in plants, protecting the protoplasm (which is able to build itself up from carbonic acid, water, and 

 ammonia under the action of sunlight) from the destructive effect of the violet rays ; the parasitic 

 alga? are probably of great service to the Sponge, both in absorbing its excreted carbonic acid, and 

 liberating oxygen for its use. 



The Canal System. Although the type of canal system described in Euspongia is by far the 



Fig. 8. 



PRI MORDIALIS, 



x 50 dia. (After 

 Haeckel.) 



