152 NATURAL HISTORY. 



grow and lead a free life, return to the parental shape. In the caterpillar, which lives to eat, the 

 chrysalis, which is stationary and quiescent, and the free-flying moth, which may never touch food, the 

 metamorphosis is very complicated ; in others it does not occur at all, the young being born the 

 image of their parent. Now, in all these instances the covering differs at different periods of their life, 

 and moulting of it is frequent. Beautiful hairs, scales, and other modifications of the skin occur, and 

 the colour is often striking and changeable. On the other hand, a colourless body may exist furnished 

 with slime cells. Again, in some of the groups the skin is modified into stinging organs, as in the 

 Jelly-fish and Coral. Muscles are attached within to these outside structures, which are thus of as 

 great importance to the Invertebrata as the skeleton is to the Vertebrata. 



The breathing, whether in air, fresh or salt water, takes place by the action of the whole, or part 

 of the whole skin, by parts of it which are arranged and ciliated like gills, or which are turned 

 inwards, like sacs and tubes within the body. Either the process is simple, the air or the aerated 

 water coming in inevitable contact with the skin and its modifications, or, as in the Insecta, the move- 

 ments of the segments of the body expel and draw in air. The circulation is carried out by contractile 

 vessels in the higher groups, but none of them have the simplest vertebrate heart, although that of 

 Atuphioxus is imitated to a certain extent. In some the current of blood can be reversed. A great 

 nunibe'r have no organs of circulation, the juices pouring from part to part in an almost plant-like 

 manner. The coloiirless blood is often without any corpuscles. A system of water channels and 

 spaces often exists. The nervous system may be greatly concentrated in the head, and it then is 

 situated above and below the gullet, there being branches on either side. A long cord with swellings, 

 or ganglia, passes from the brain along the inside of the lower or ventral side of the body in a vast 

 number of genera. In others the nervous system is supplied to the principal organs and foot in an 

 unsymmetrical and irregular manner, and in the lower groups, whose construction is simple, the; 

 nervous element is extremely difficult of demonstration, and may radiate from centres, pass round 

 the body, giving off threads to special organs, or may merge here and there into muscular struc- 

 tures, there being nothing like a nervous centre. No structure comparable with voluntary nerve 

 fibre within is visible in the simplest forms of the Invertebrata. 



Some of the Invertebrata have organs of special sense faintly developed, in comparison with those 

 of the Vertebrata. Simple and compound eyes, or mere spots of pigment in contact with nerve, are 

 common, but many groups are without any special structures by which light can be distinguished, 

 although the influence of it is evident enough on their bodies. In one group the eye is internal 

 and useless during a part of their life. Rudimentary but most useful organs of hearing exist in 

 some ; tactile nerve is exceedingly delicate in many ; and a knowledge of the presence of food, or 

 of substances, which give an impression of disgust to man, is evident enough in so many kinds, that 

 something analogous to the sense of smelling must be present in them. 



Many Invertebrata exist in very cold water, others live in warm brine springs, some require the 

 purest air or the purest water, whilst not a few which are parasites live in impure situations. The 

 intelligence and constructive acts of many Invertebrata are as evident as the simple, mechanical, and 

 automatic lives of others ; and it does not appear that it is possible to connect the highest 

 intelligence with the highest development of the body generally in any scheme of classification. 

 Moreover, the kind of intelligence differs in the different phases of the lives of many of the Inverte- 

 brata, There are many instances in which care is taken of the young by the parent or by the com- 

 munity, but in the majority this is not the case. 



The methods by which the Invertebrata increase and multiply are numerous and extraordinary. 

 Spontaneous division of the body, in one or more pieces, each becoming a separate animal ; separation 

 of the tissue of the whole creature into a vast number of minute microscopic globules, which burst 

 forth and grow into the parent shape ; separation of little pieces from the outside or inside, these 

 becoming independent ; a process resembling internal budding ; the formation of living young within 

 the body, which pass forth not in the egg, but resembling the parent ; and the laying of eggs, 

 are the commonest. But the results of the egg-laying are as extraordinary as the other methods. 

 Some eggs are produced by virgin mothers, and in the hatching of all eggs there is a process of 

 evolution within the egg envelope or shell. Most young thus produced do not resemble the matured 

 parent, and pass through different stages of existence and shape before attaining maturity ; and whilst 



