706 



SCIENTIFIC RECREATIONS. 



of these shells, and is practically a chalky deposit ; the shells are being 

 built up as in former ages with the curious nummulites o/ the Eocene for- 

 mation, which are amongst the most interesting of fossils. 



SPONGES. We must go on at once to the Sponges, which form such 

 an interesting subject as they are so familiar to us all. Sponge is not often 

 regarded by the public as an animal, and though perhaps authorities may 

 not have yet concluded in what category they should be placed, we may 

 consider them here according to the list. 



We find the spongida both in fresh and salt water, and they have given 

 rise to much discussion as to whether they should be classed as animals 

 at all. But that question having been finally settled, we can proceed to 

 examine a sponge in its native state, and we shall find both skeleton and "flesh." 

 The skeleton is hard and composed of needles of " tiny " texture. 

 The flesh is " sarcode," and the animal possesses no mouth, but is full of 

 holes (pores) and canals through which the water is continually distributed. 

 The outer layer of the sponge is formed of ultimate components of the living 

 substance of the sponge (like the amebae we have been considering). Each 



contains a nucleus, and when joined together form 

 the outer layer of the body. Beneath is a wide 

 cavity communicating with the exterior by means of 

 minute holes, and filled with water. The cavity 

 separates the superficial layer from the deeper sub- 

 stance, which is of the same character. In the 

 water passages of the sponge are cilia which induce 

 Fig. 829.-Fragment of sponge a cement, and the interior canals develop into cham- 



(magnified). 



bers lined with sponge particles, and the water 



carries particles to the sponge, which represents a kind of sub-aqueous 

 city, where the people are arranged about the streets and roads in such 

 a manner that each can easily appropriate his food from the water as it 

 passes along. * 



Sponge, then, is a mass of living organisms tiny living creatures 

 capable of feeding and of movement, resembling amebse or perhaps infusoria, 

 with cilia, to enable them to obtain nourishment by a kind of inhalation 

 or respiration. They are reproductive by sexual and a-sexual processes 

 which produce spongellce. The living sponge is a beautifully coloured 

 animal, and grows upon almost any solid foundation ; and in the autumn 

 the parent sponge displays a number of yellow dots or " gemmules," which 

 are the young. These are soon cast off and left to shift for themselves, 

 and seek their fortunes, helpless as they appear, in the wide and stormy 

 sea. At last they find a resting-place, and fix themselves for ever, growing 

 up and reproducing their species until they are carried off to be sold and 

 used in civilized countries for domestic purposes. 



We must leave these curious animalculae and glance at the INFUSORIA 



* Huxley. " Classification of Animals." 



