MARINE ARTHROPODS 



257 



cords, and from which nerve fibres are distributed to the various 

 parts of the body. It should be noted, however, that some members 

 of the group have degenerated into parasites, and in these, as with 

 all such degraded creatures, many of the organs have retrogressed 

 to such an extent that they are quite functionless, or have even dis- 

 appeared entirely. These parasitic forms, when very young, are 

 really highly organised creatures, not unlike the young of their 

 industrious and more noble relatives ; but, as the natural result of 

 their degraded mode of living, in which they find no use for their 

 organs of locomotion, digestion, 

 circulation and respiration, these 

 eventually disappear, with the 

 result that the organs of repro- 

 duction predominate to such au 

 extent that they often fill the 

 greater part of the cavity of the 

 body. 



It should be noted, too, that 

 the sense organs of arthropods 

 are well developed, most of them 

 being supplied with complex 

 eyes, hearing organs, and highly 

 sensitive feelers. 



This sub-kingdom consists of 

 four classes the Crustacea, in- 

 cluding lobsters, crabs, shrimps, 

 prawns, &c.; Arachnoidea, con- 

 taining spiders, mites, and scor- 

 pions ; Myriopoda centipedes 

 and millepedes ; and Insecta. 



The first of these classes con- 

 sists mainly of marine animals, 



and will therefore occupy much of our attention, but the members of 

 the other three are mostly terrestrial and aerial creatures that do 

 not fall within the scope of this work, except in the case of a few 

 species that are more or less decidedly marine in their tendencies, 

 The aquatic members are generally provided with well-formed gills 

 by means of which they are enabled to extract the dissolved oxygen 

 from the water in which they live, while those of terrestrial and 

 aeiial habits breathe by means of a system of tracheae or air-tubes 

 that are open to the air and supply branches to all parts of the body. 



Fio. 188. THE NERVE-CHAIN OF 

 AN ABTHKOPOD (LOBSTER) 



>, optic nerve ; c, cerebral ganglion ; 

 i, large ganglion behind the oesophagus; 

 (A, ganglia of the thorax ; ab, ganglia 

 of the abdomeu 



