Evolution 



which is followed by an intestine. Salivary glands and a large 

 hepatic gland or liver are present. Respiration occurs partly 

 through the skin, but special organs also exist for this function, gills 

 in the water forms, and a lung cavity in those which breathe air. 

 There is a well-developed blood system, and generally a heart ; the 

 blood is pumped direct from the heart to the general body tissues, 

 and returns to it by way of the kidneys or nephridia, which purify 

 it of waste materials, and the respiratory organs, where it is freed 

 of carbon dioxide and supplied with oxygen. The nervous 

 system varies greatly, but a pair of cerebral ganglia a brain- 

 is usually present. There is a particularly keen sense of smell, 

 and taste and hearing may also easily be shown to exist. Some 

 forms are blind, from which condition there is a regular series of 

 stages of development of the eye, up to forms in which it becomes 

 a highly perfected organ, with cornea, iris, lens, and retina. The 

 close similarity between this and the ordinary vertebrate eye, 

 which must have evolved quite separately, is one of the strangest 

 coincidences of Evolution. Thus in many ways the molluscs are 

 to be regarded as highly specialised types. But in two important 

 directions, in intelligence and in their arrangements for locomo- 

 tion, they stand as a group on a low plane of development. 

 Figs. 49, 50, and 51 illustrate some of the forms met with in 

 the group. The origin of the molluscs, as well as that of the 

 Echinoderms, is wrapped in obscurity. That each group is 

 derived from some form of worm is probable, yet some zoologists 

 hold even such a general statement as this to be lacking in 

 support. 



Our third great group is that of the Arthropods (literally 

 'jointed footed'), including the Crustaceans (crabs, lobsters, 

 shrimps, etc.), spiders and mites, centipedes and insects. The 

 Arthropods are sometimes classed together with their ancestors, 

 the ringed worms (such as the common earth-worm), as Articulata, 

 a name which refers to a very obvious feature, the repetition of 

 similar segments in a regular series from front to rear. This 

 is perhaps most apparent in the ringed w r orms and centipedes, 

 but it is to be seen in all members of the group. This same 



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