INTRODUCTION. 



great importance when viewed from a systematic standpoint. Beyond the maturation 

 and segmentation of the egg the Arthropod have but very few embryological features 

 in common ; on the other hand, the evidence presented by the development of the 

 insects and Crustacea is such as to be almost conclusive of the distinctness. We have, 

 however, one feature to note in connection with their growth. As we have seen, the 

 Arthropoda are enclosed in a chitinized integument, which forms a firm investment for 

 the body, without provision for any increase in size, to correspond with the natural 

 growth of the animal, and hence, at intervals more or less frequent, according to the 

 size and rapidity of growth, the integument, or rather its outer hardened portion, is 

 shed, and then the underlying skin produces a new and larger external skeleton, to be 

 shed in turn when further increase in size renders it necessary. In connection with 

 this growth in size and consequent shed- 

 ding of the skin or exuviation most of the 

 Arthropods undergo marked metamorphoses, 

 the result being to produce forms widely 

 differing from the younger stages. Some- 

 times the changes are effected gradually, 

 slight differences being noticeable at each 

 moult, at other times the differences be- 

 tween two moults being enormous, as shown 

 in Plate I, which repi-esents the changes 

 produced by a single moult in the edible 

 crab (Neptunus hastatus) of the Atlantic 

 coast. The only other example which need 

 here be instanced is that presented by the 

 Butterflies, familiar to all, in which a worm- 

 like larva hatches from the egg, eats and 

 grows, occasionally casting its skin to accom- 

 odate its increase in size, but without much 

 changing its general appearance, until at 

 last, by a single moult, it passes suddenly into 



a chrysalis, which presents but a very slight resemblance to the previous larval condi- 

 tion. A period of apparent inaction now intervenes, in our climate frequently lasting 

 through the winter, but beneath the skin of the chrysalis great changes are in progress, 

 and in due season the skin is shed again and for the last time, and the perfect form, the 

 butterfly, is the result. In the pages of this volume many examples of these metamor- 

 phoses will be found. 



The Arthropoda are usually divided into two classes, Crustacea and Insects, but in 

 nature we do not find such exact classifications as are to be found in books, and there 

 exist many groups of Animals which do not readily fit in any of the accepted classifi- 

 cations. Among the Arthropoda we find such forms, whose position is by no means cer- 

 tain, and which have alternately been regarded as belonging to the Crustacea and to 

 the Insects. Owing to this unsettled condition of our knowledge and opinions, in the 

 present work the Horse-Shoe Crabs and Trilobites, the Water-bears, Sea-spiders, and 

 Linguatulina are placed between the Insects and Crustacea, where it is possible they 

 really belong. 



J. S. KlNGSLEY. 



EIG. 4. Ear of grasshopper, from within, a. Thick- 

 ened rim of (6) tympanum, c. Muscles, d. Spir- 

 acle, e. Conical process, fc. Triangular chamber. 

 /. Auditory nerve, m. Auditory ganglion. 



