ARTHROPOD A. 87 



respiratory tubes known as Trachece may be elaborated (Insecta); 

 whilst the Onistacea or Crabs, Lobsters, &c., are provided 

 with branchise or gills, being aquatic in habits. Development 

 may or may not be direct. Spiders (Araneida) are very similar 

 to the adult when first hatched. Insecta and many Crustacea 

 develop by means of a metamorphosis or transformation : the 

 young insect or crustacean 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 the 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 Arthropod rupturing and then re- 

 leasing the larva, which has prior to its moult formed a new and 

 soft skin, capable of distension, beneath. This enables the in- 

 ternal organs to swell out, and thus growth takes place. During 

 this eedysis the entire exoskeleton is cast, even the chitinous 

 covering of the eyes and feelers. Growth also takes place be- 

 tween 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 metamorphosed into the imago by a process called Hisiolysis. 

 Sense-organs are well developed in most arthropods ; organs of 

 vision are represented by two kinds of eyes, — simple eyes or 

 ocelli, and compound eyes or facetted eyes, complex ocular 

 structures peculiar to the jointed-limbed animals. The senses of 

 hearing, taste, and smell are also strongly developed, the an- 

 tenna or feelers being probably the most important structures 

 in connection with hearing and smell, although the palpi pro- 

 bably 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-chord 

 and a pair of ganglia in each segment. Considerable variations 

 take place in the latter : numbers of ganglia may amalgamate, as 



