POKIFERA.- 



-RADIATA- 



-Cai.carea. 



405 



only a sort of honiy skeleton, on which the true living ' 

 portion of the sponge is supported. This consists of 

 a gelatinous matter, wliich is spread over all the fibres 

 of the reticulated skeleton ; its consistence is very like 

 that of the white of an egg, and it runs freely away 

 from the sponge when the latter is taken out of the 

 water. This gelatinous substance resembles that of 

 which the Ainwba, one of the Rhizopoda, is composed, 

 am! a parently consists of minute masses of aggregated 

 sarcode cells. Each of these cella appears to possess 

 an independent existence ; those on the surface being 

 furnished witli long and very slender vibratile cilia. 



If we take a piece of common sponge, we see that 

 its surface is every where perforated with an infinite 

 number of minute holes, amongst which a considerable 

 number of large openings are scattered. When this 

 sponge is examined in a living state, a rapid stream 

 of water may be observed issuing constantly from these 

 larger orifices. This excurrent stream of water is ren- 

 dered observable by the fact, that it bears with it a 

 number of minute particles from the interior of the 

 sponge. This water is imbibed through the minute 

 pores distributed in such profusion over the entire sur- 

 face of the sponge. After passing through these, and 

 traversing the cavities formed in every direction by 

 the reticulated structure of the mass, it is collected into 

 canals, by which it is finally conducted to the larger 

 openings of the surface. The objects of this continual 

 flow of water are for nutrition, for the expulsion of 

 digested particles, for respiration, and assisting in the 

 propagation of the species. 



The propagation of the species is effected in various 

 ways. They are propagated by division ; when cut 

 io two, the two portions will each grow and become 

 perfect sponges. They are also, and more usually, 

 propagated by gemmules, which sprout from the deli- 

 cate granular film covering their skeleton, and which, 

 as they gradually become detached from the parent 

 body, are furnished with cilia, swim freely about, like 

 infusorial animalcules, and afterwards, finding a favour- 

 able locality, become fixed and eventually develope into 

 perfect sponges. In a third case, propagation takes 

 place by seedlike bodies being produced in the sub- 

 stance of the sponge. At first these consist of several 

 cells merely united together into a globular or ovoid 

 mass, lying freely in cavities of the substance of the 

 sponge. This mass of cells then becomes enveloped 

 in a capsule. When these cells are pressed out of the 

 cavity of the capsule under water, they soon swell up 

 and burst, and the germs contained in them gradually 

 become dilTused over the bottom of the vessel. In 



a few days these collect into separate groups, each 

 inclosed in a mucilaginous substance. From these 

 germs active animacules are produced, possessing a 

 power of locomotion by the extension of ditferent 

 parts of the body. They exhibit considerable acti- 

 vity, asume at times fantastic forms, and are very 

 voracious, absorbing other minute animals into their 

 substance, in the same manner as has been observed 

 in the Amoebse amongst the Rhizopoda. 



Sponges vary much in form, and are for the most 

 part fixed by a kind of root at the base, and attach 

 themselves to almost every body which can afford tliem 

 support, whether fixed or floating. Many are found 

 encrusting submarine objects, as stones, shells, rocks, 

 &c. They are mostly marine, only comparatively few 

 being found in fresh water. In the living state, many 

 of them possess lively colours, and usually grow in 

 groups. They occur in all seas, from the equator to 

 the pole. A peculiar group of sponges, belonging to 

 the genus Cliona, are peculiar for the habit they have 

 of boring. Shells, other submarine bodies, and even 

 rocks, are penetrated by these unobtrusive-looking 

 animals ; and they elfect a powerful action upon these 

 bodies by slowly and silently producing complete dis- 

 integration. There are three kinds of sponges, each 

 of which is composed of many species, and form as 

 many orders or divisions : — 



THE HORNY SP0N6JES {Cornea) are distinguished 

 by having uo spicula, and have the body very porous 

 and elastic, soft, and composed of a fibro-corneous 

 skeleton, which anastomoses in all directions. 



Genus Spongia. — This genus is the type of this 

 group ; and in it is contained the common sponge of 

 commerce. The sponges of commerce are obtained 

 in considerable abundance from the Mediterranean, 

 and are usually prepared for use by soaking them in 

 dilute hydrochloric acid, in order to remove all traces 

 of lime, and then bleached and well beaten to render 

 them fit for market. The inhabitants of some of the 

 Greek islands have been trained from their infancy to 

 dive for these substances. They adhere firmly to the 

 bottom, and are not detached without a great deal of 

 trouble. Smyrna is the great market for sponge. 



THE SILICEOUS SPONGES (Silicea) have the body 

 more or less rigid or friable, and strengthened with 

 siliceous spicula. The group is represented by the 

 genus Thethys. 



THE CAIiCAEEOUS SPONGES (Calcarea) have the 

 body, which is not very soft, and composed of a sub- 

 cartilaginous substance, supported by calcareaus spicula. 

 The genus Granlia is thf type. 



•f.XT) OK RADIATA 



