98 ARTHROPODA. 



respiratory tubes known as Trarhen- ma,y be elaborated (Hexa- 

 poda, &o.) ; whilst the Cnistacea or Crabs, Lobsters, Wood-lice, 

 &c., are provided with branchise or gills. Development may or 

 may not be direct. Spiders (Aranuida) wee very similar to the 

 adult when first hatched. Hexapoda and many Crustacea 

 develop by means of a metamorphosis or transformation : the 

 young insect on hatching from the egg is totally unlike the 

 adult or imago, and is known as the larva. The larval stage 

 may give rise to a second condition, the pupa or chrysalis, in 

 some insects. The changes from the larva to the imago or adult 

 take place partly by a series of moults or ecdyses — the old skin 

 of the insect rupturing and then releasing the larva, which has 

 prior to its moult formed a new and soft skin, capable of dis- 

 tension, beneath. This enables the internal organs to swell out, 

 and thus growth takes place. During this ecdysis the entire 

 exoskeleton is oast, even the chitinous covering of the eyes and 

 feelers. Growth also takes place between these " moultings," 

 for there is a soft space between each segment or somite which 

 enables the entire abdominal region to stretch. There are in 

 some insects, however, more complicated changes than those 

 mentioned here. A complete remodelling of the larval body 

 takes place in the pupal state, whereby the larva is metamor- 

 phosed into the imago by a process called Hisioh/siy. Sense- 

 organs are well developed in most arthropods : organs of vision 

 are represented by two kinds of eyes, — simple ej'es or ocelli, 

 and compound or facetted eyes, complex ocular structures 

 peculiar to the jointed-limbcd animals. The senses of hearing, 

 taste, and smell are also strongly developed, the antennffi or 

 feelers being probably the most important structures in connec- 

 tion with hearing and smell, although the palpi probably have 

 some such function. In conjunction with these we find a highly 

 developed brain, at least for invertebrate animals, and a central 

 nervous system consisting of a ventral nerve-cord and a pair of 

 ganglia in each segment. Considerable variations take place in 

 the latter : numbers of ganglia may amalgamate, as seen in some 



