53 



PART SECOND. 



In this part the animal world is described briefly in its natural 

 order and not according to the more or less accidental arrangement of 

 the tanks in the aquarium. 



Since there are many animals in the aquarium, which in their 

 appearance and habits so closely resemble plants as easily to be mistaken 

 for such , it may not be out of place to devote a few words to the 

 explanation of the 



Differences Between Animals and Plants, 



The Plants, adapted to live on gases and salts, and the Animals, 

 adapted to live on vegetables and on each other, are supposed to have 

 arisen as two well-defined groups out of the lowly forms of earliest life. 

 The earliest living things cannot have been exactly animal, for there was 

 nothing to eat; but they need not have been exactly vegetable. Among 

 the microscopic organisms even now existing there are several which 

 many biologists hold to be not properly classed as either animal or 

 vegetable; an instance of such are the much talked-of Bacteria. 



But of the beings of larger size, such as those treated of in this 

 guide, there are none whose nature is doubtful. It is a common mistake 

 to suppose that Corals, Anemones, tube-inhabiting Worms, Sponges and 

 Sea-squirts are half-way between plants and animals. They are all quite 

 unequivocal animals. They are rooted like the plants, but whereas a 

 plant supports an easy and indolent existence on the air, water and 

 sunlight which compass it, with the mineral salts that soak into it, a 

 rooted animal is not necessarily any less active than a free-swimming 

 one. Not only the Corals, Tube-worms, and Sea-squirts, but bivalve Mol- 

 lusks such as the oyster, fixed Crustaceans such as the barnacle, the 

 sedentary though not fixed lancelet (a low Vertebrate, see p. 87), with 

 many others, obtain their food like the Sponges by actively causing cur- 

 rents to pass through them, or over their surface, and filtering from the 

 water the nutritious particles it contains. The current is generally caused 

 by minute vibrating hairs. It will be seen that for the purpose of ob- 

 taining food, it is just as efficient and quite as much exertion if, in- 

 stead of the animal passing through the water, the water passes through 

 the animal. 



A fixed career requires little intelligence, and we consequently find 

 all the different animals which have so settled their lives characterised 

 by the possession of much less brain, or its equivalent, than their roving 

 relations. But they have well-developed muscles and digestive organs, 



