2 NATURAL HISTORY OF ARTHROPODS. 



admit of a freedom of motion. In many cases, as in most insects, it is found that all 

 traces of these appendages have disappeared from some of the body segments, though 

 where we examine the larv£e or immature stages, we find that the generalization is fully 

 justified, and that in the most generalised types each of these segments also bears a pair 

 of limbs. This segmentation of the body and appendages is almost entirely confined 

 to the external portions, the nervous system alone exhibiting a similar character, and, 

 though constant throughout the group, it seems to be a secondary feature, and the re- 

 sult of a provision for movement rather than a fact of great morphological importance. 

 This external envelope of the body is of cutaneous origin, and is rendered firm and 

 hard by a peculiar organic substance known as chitine. This chitine, first made known 

 by Odier, resembles the cellulose of plants in not being dissolved in caustic potash, but 

 it differs essentially from it in containing nitrogen. In addition to this chitine there 

 are frequently present in the exo-skeleton salts of lime, calcic-phosphate and carbonate, 

 which render it much harder, and consequently more of a protection to the animal. 



The rings of the body and the appendages which they bear, are variously modified 

 according to the parts they have to protect' and the functions they have to perform. 

 Some of the appendages are adapted for walking, some for swimming, some for the 

 seizure and mastication of food, while others play a part in the respiration, and still 

 others give support to organs of sense. Of these appendages and their structure we 

 shall speak more in detail in the succeeding pages. 



In their internal structure the Arthropoda agree in many important particulars, but 

 it is to be noted that in many respects these characters are common to other groups of 

 Invertebrata, a fact which renders them of less weight in defining the Bi-anch Arthro- 

 poda. The heart is usually an elorigate tube on the dorsal surface of the body, is usu- 

 ally provided with valves, and serves for the propulsion of the blood, generally in a 

 direction from behind forward. The arteries have proper walls, but the venous system 

 consists merely of spaces or laciindB between the various organs. The blood is usually 

 colorless, but occasionally is yellow or red, or even purple, but the color belongs to the 

 fluid itself, not to the contained globules. 



The principal nervous system consists of a series of ganglia, or nervous centres, 

 nomially one to each segment, connected by a double longitudinal cord. This corre- 

 spondence of ganglia and segments is the most evident instance of a segmentation of 

 the internal organs. The first ganglion or " brain " lies in front of the mouth, and 

 from it arise the nerves going to the eyes and antennse. The two nervous cords con- 

 necting it with the rest of the nervous system pass one on either side of the CBSophagus 

 and enter the first of the series of infra-CESophageal ganglia. Thus we see that the ali- 

 mentary canal passes through the nervous system, a feature which has its analogies, if 

 not homologies, in other groups of the animal kingdom, and notably among most of the 

 worms and molluscs. The posterior portion of the chain lies on the floor of the body- 

 cavity, and, as has been said, normally consists of a series of ganglia, one to each seg- 

 ment, but frequently some of the ganglia are fused together, and where, theoretically, 

 there should be several ganglia but one compound one is found. Each ganglion gives 

 off nerves to the adjacent organs, and where we find two or more ganglia united, the 

 nerves usually remain separate, thus clearly showing just what has taken place. A sec- 

 ondary nervous system is frequently well developed, analogous to the sympathetic 

 system of the Vertebrata, and with a somewhat similar distribution. 



The alimentary canal is usually nearly straight, traversing nearly the entire length 

 of the body, and for the greater part of its course lying between the nervous and cir- 



