CLASSIFICATION OP THE ANIMAL KINGDOM. 119 



different construction, are very little known. Some secrete calcareous tubes, 

 and others clothe themselves with shells, grains of sand, and other matter. 

 Their organs of respiration and other appendages are placed near the 

 head. 



We now come to the second class of this division — the Myriapoda; and 

 here we find animals capable, from the greater density of their external 

 skeletons, to subsist on land. Their bodies consist of a series of segments, 

 to each of which is appended one or two pairs' of articulated legs; the 

 anterior segment or head being provided, besides the mouth, with eyes and 

 jointed antennae. They breathe through spiracles, and the air is distri- 

 buted internally by means of trachea. The segments of the body, and 

 consequently the feet, increase in number with age, thus differing materially 

 from insects. They are divided into two great families — the Zulidce or 

 Millipedes, and the Scolopendridce or Centipedes; the former most nearly 

 assimilate with the last class, the Annelidans. Their bodies are long and 

 round; the feet, of which there are two to each segment, though articu- 

 lated are yet very small, so that these animals cannot move very quickly; 

 their mouths resemble those of the larvae of some insects, while, like the 

 Annelidans, they have a straight and very capacious alimentary canal. 



The internal organization of the class Myriapoda is also similar to that 

 of insects, while in other respects they materially differ; as, for instance, 

 in the position of the sexual organs, which, in the animals we are 

 considering, are situated at the anterior part of the body, like the 

 Annelidans; while, among insects, they are invariably placed at the caudal 

 extremity. Again, they differ materially in their growth and development. 

 Perfect insects, that is, insects that have completed all their changes, have 

 six legs, and are then capable of reproduction; while the Myriapods acquire, 

 by the changes which they undergo, new segments and legs, and cannot 

 propagate their species till two years after their last moult. 



The second family of this class, the Scolopendridoe, are more highly 

 organized than the Millipedes, and unite still further the Annelidans with 

 the class Insecta. Their segments are fewer in number, flat instead of 

 round, and more horny and tough than those of the last family; their 

 legs also are larger and more pliable. Their nervous system is of course 

 more concentrated and developed. Their mouths are very strong, and are 

 armed with sharp curved fangs, perforated at their extremity. The alimen- 

 tary canal is smaller than that of the Millipedes, in proportion to the 

 nature of their food; the Millipedes living on vegetable substances, while 

 these animals feed on animal matter. Their respiratory and circulating 

 system seems to be like those of insects; and they also resemble that class 

 in the position of their sexual organs, which, unlike the Zulidce, are situated 

 at the extremity of the tail. 



