14 
Tank No. 21. 
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 plantsbut 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 Mollusks 
such as the oyster, fixed Crustaceans such as the barnacle, the sedentary 
though not fixed lancelet (a low Vertebrate, see p. 66), with many others, 
obtain their food like the Sponges by actively causing currents 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 obtaining food, 
it is just as efficient and quite as much exertion if, instead 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, 
and with all their innocent and flower-like beauty, lead not only active 
but predatory lives. For the most part it may be affirmed with certainty 
that they are the descendants of freely swimming animals which have 
preferred security to independence and monotony to danger. 
Roughly speaking, the differences between plants and animals are as 
follows. Few, if any, plants are known to digest solid food; almost all 
animals do so. Few, if any, animals have the peculiar green colour 
which enables a plant in the sunlight to feed on air; most plants possess 
it. — The great majority of plants derive their strength from a suppor¬ 
ting honeycomb of cellulose (the substance of which cotton is composed) 
or allied woody matters containing no nitrogen; these have been met 
with in the animal kingdom only among the Tunicates (see p. 62). Most 
animals possess supporting structures of the nature either of horn or 
gristle, containing much nitrogen; these substances have not been found 
in plants. — Plants rarely show any strong power of movement, animals 
are rarely without it. — Last, but not least, an animal shows signs 
of relationship to other animals, and a plant to other plants. With the 
exception of the occurrence of cellulose in those most undoubted animals, 
the Sea-squirts, there is not one of these characters in which any of the 
creatures above referred to approach the plants. 
It is worth adding a word to say that the old conception of the ”Vege¬ 
table” as a half-way house between "Animal” and "Mineral” is very 
delusive. The structure of the final living matter in plants and animals 
appears almost identical, and all the differences which can be enumerated 
between them sink to nothing beside the gulf that separates both from 
non-living matter. 
