PROTOZOA. 169 



(Silicea) sometimes have their spicules woven or fused 

 together, as in the beautiful Euplectella, or Venus's 

 Flower-basket. In Hyalonema, the glass-rope, the long 

 spicules, are twisted together. 



In the order Calcarea the skeleton is composed of Car- 

 bonate of lime. Except a few fresh-water species, as 

 Spongilla, sponges are marine. The best sponges of 

 commerce are from the Mediterranean. 



7. The colonies of bioplasts in Thalassicollida and in 

 Sponges are analogous to the higher types of animal life, 

 yet the individual cells are so loosely bound together, 

 and so capable of living and performing all their func- 

 tions apart, that they are ranked as Protozoa, as the col- 

 onies of Volvocineae, Nostochaceae, and Confervaceae, are 

 placed among the Protophytes. 



8. The essential difference in the vital powers of dif- 

 ferent classes of living things, and of the individuals of 

 each class, is well exhibited in the following passage from 

 Johnston's " British Sponges :" 4< For example, it is very 

 common to find growing on the same rock, or sea-weed, 

 a silicious, a calcareous, and a horny sponge ; they have 

 all the same exposure, and are all recipients of the same 

 nutriment, yet does each act upon this differently. One 

 extracts from the fluid silica, which it causes to assume 

 a solid crystalline form ; another selects in the same 

 manner the calcareous particles, which, obedient to the 

 laws of life, assume figures novel to them in their min- 

 eral state ; and again, another rejects both the lime and 



the flint as injurious to its constitution." 

 15 



