184 



THE POPULAR EDUCATOB. 



with the fluids of the body on one (internal) side, and oxygen 

 on the outer side. These are the necessary conditions of 

 respiration, and therefore of life. Now the water contains a 

 sufficient amount of oxygen for the purposes of respiration 

 dissolved in it, and the other condition namely, the moisture of 

 the membrane which contains the nutritive fluid of the body is 

 maintained in the water-animal without any contrivance what- 

 ever. Hence the exterior of the body, or a lobe or leaflet 

 protruded into the water around, is quite sufficient to enable 

 water-animals to breathe. On land it is different. The higher 

 animals must have elaborate contrivances to maintain the 

 moisture of the respiratory membrane. It must be placed 

 internally, lest the external air and wind should carry off the 

 moisture. It must be confined to small cavities, lest their large 

 capacity should incommode the animals, and being thus limited 

 the membrane must be folded elaborately to increase its area. 



in the air, but many of them pass their earlier stages, when 

 they are feeble and need protection and easy conditions of life, 

 in the water. The crust of the earth contains multitudes of 

 animal remains, but the aquatic forms outnumber the aerial in 

 an almost unlimited proportion. Further, the first forms found 

 in the earliest strata are water-animals, and we have good 

 reason to believe that fish existed before reptiles, birds, or 

 brutes. 



In conformity with the preceding remarks we find and have 

 found that, in tracing upward the grades of the animal kingdom, 

 we have not yet arrived at any animals suited for an aerial 

 existence. Their parts are not sufficiently differentiated for 

 such a life. With regard to many of the Coelenterata, if 

 placed where the water drains away from them they fall to 

 pieces, or sink into a semi-fluid slimy condition, never to be 

 restored to their original form. 



II. 



I, CARYOPHYLLIA SMITHII, A DEVONSHIRE COAST ANIMAL. II. DRY CORAL OF CARYOPHYLLIA SMITHII. III. DIAGRAMMATIC SECTION, SHOWLXG 



HOW RED CORAL IS SECRETED. IV. CESTUM VENERIS (VENUS'S GlRDLE), A ClENOPHORE. V. ONE OF THE POLYPES OF ALCYONARIA. 



la animals where these contrivances are not found, or not 

 found in efficient condition, life in the air is difficult to maintain. 

 Such animals are always in danger of being dried up. Thus 

 .he toad must keep to his dark, moist hole. The grey slug 

 aever comes out but at night, and the black slug only after 

 rain. It is, in fact, scarcely too much to say that the water is 

 both the home and the cradle of life. Not only are all the 

 lower animals aquatic, but the lower forms of many of the 

 higher classes are so too. Both zoology and geology proclaim 

 this fact. Life teems in the ocean. Its countless myriads of 

 forms people the main and crowd up even to the coast-line, 

 despite the dangers of the beach. Every sweep of the 

 entomologist' s t water-net in a fresh-water stream takes some 

 living thing, and every drop of water contains countless 

 animals. Though Nature is redundant of forms everywhere, 

 yet this could scarcely be said of earth or air. With extreme 

 difficulty do animated forms seem to have made conquest of 

 the earth and air. Their mother country, their arsenal where 

 they prepared and armed themselves for the expedition, was the 

 water. Insects, more than any other living things, are at home 



The type of the class Actinozoa, which occupies the same 

 relation to the rest of these animals that the simple hydra 

 does to the Hydrozoa, is the common sea-anemone (Actinia 

 mesembryanthemum). This animal has already been described, 

 and its structure may be so well understood by looking at the 

 diagram of its vertical and horizontal sections given in the 

 illustration attached to the last lesson, that we need not refer 

 to it further. 



The common sea-anemone is wholly soft, but some of its 

 near allies exhibit a tendency whose results are very complicated 

 and interesting. This tendency is to deposit either externally 

 or in the substance of their tissues carbonate of lime, which, 

 being of the same nature as marble, is hard and enduring. 

 This encrustation forms both a protection and a support to 

 the otherwise soft animals, so that they can not only endure 

 but enjoy the buffeting of the great surface billows of the 

 ocean. Actinozoa of this kind are not so common in England 

 as in the tropical seas, but the Devonshire coast furnishes the 

 little coral-secreting animal represented in the engraving (Fig. I.). 

 When this tendency to deposit a hard structure of carbonate 



