164 THE RIDDLE OF THE UNIVERSE. 



the metazoa; from it, in all the nerve-animals, not 

 only the external skin and the organs of sense, but also 

 the nervous system, are developed. In the gastrseads, 

 which have no nerves, all the cells which compose 

 the simple epithelium of the ectoderm are equally 

 organs of sensation and of movement ; we have here 

 the tissue-soul in its simplest form. 



The platodaria, the earliest and simplest form of 

 the platodes, seem to be of the same primitive con- 

 struction. Some of these cryptocoela — the convolute, 

 etc. — have no specific nervous system, while their 

 nearest relatives, the turbellaria, have already differen- 

 tiated one, and even developed a vertical brain. 



The sponges form a peculiar group in the animal 

 world, which differs widely in organisation from all 

 the other metazoa. The innumerable kinds of sponges 

 grow, as a rule, at the bottom of the sea. The simplest 

 form of sponge, the olynthus, is in reality nothing 

 more than a gastrcea, the body-wall of which is per- 

 forated like a sieve, with fine pores, in order to permit 

 the entrance of the nourishing stream of water. In 

 the majority of sponges — even in the most familiar 

 one, the bath-sponge — the bulbous organism con- 

 structs a kind of stem or tree, which is made up of 

 thousands of these gastrseads, and permeated by a 

 nutritive system of canals. Sensation and movement 

 are only developed in the faintest degree in the 

 sponges ; they have no nerves, muscles, or organs of 

 sense. It was, therefore, quite natural that such 

 stationary, shapeless, insensitive animals should have 

 been commonly taken to be plants in earlier years. 

 Their psychic life — for which no special organs have 

 been differentiated — is far inferior to that of the 

 mimosa and other sensitive plants. 



