Arthropoda are not known to be ciliated in any part of the body, or in any stage of growth. Another histological peculiarity, not quite so universal, is the striation of the muscular fibres throughout the body. In many Invertebrates there are no striated muscles at all, while in Vertebrates only voluntary muscles, as a rule, are striated. The circulatory organs of Arthropoda vary greatly in plan and degree of complication, but there is never a completely closed circulation. The development of Arthropoda may be accompanied by striking metamorphosis, _e.g._, in many marine Crustacea, but, as in other animals, the terrestrial and fluviatile forms usually develop directly. Even in Insects, which appear to contradict this rule flatly, the exception is more apparent than real. The Insect emerges from the egg as a fully formed larva, and so far its development is direct. It is the full-grown larva, however, which corresponds most nearly to the adult Myriopod, while the pupa and imago are stages peculiar to the Insect. It is not by any process of embryonic development, but by a secondary metamorphosis of the adult that the Insect acquires the power of flight necessary for the deposit of eggs in a new site. [Illustration: Fig. 3.--Longitudinal section of Female Cockroach, to show the position of the principal organs. _Oe_, œsophagus; _S.gl_, salivary gland; _S.r_, salivary reservoir; _Cr_, crop; _G_, gizzard; _St_, chylific stomach; _R_, rectum; _Ht_, heart; _N.C_, nerve-cord. × 7.] _Characters of Insects._ Insects are distinguished from other Arthropoda by the arrangement of the segments of the body into three plainly marked regions--head, thorax, and abdomen; by the three pairs of ambulatory legs carried upon the thorax; by the single pair of antennæ; and by the tracheal respiration. Myriopods and Arachnida have no distinct thorax. Most Crustacea have two pairs of antennæ, while in Arachnida antennæ are wanting altogether. Crustacea, if they possess special respiratory organs at all, have branchiæ (gills) in place of tracheæ (air-tubes). In Arachnida, Myriopoda, and Crustacea there are usually more than three pairs of ambulatory legs in the adult. The appendages of an Insect’s head (antennæ, mandibles, maxillæ) are appropriated to special senses, or to the operations of feeding, and have lost that obvious correspondence with walking legs which they still retain in some lower Arthropoda (_Peripatus_, _Limulus_, _Arachnida_). The thorax consists of three[8] segments, each of which carries a pair of ambulatory legs. No abdominal legs are found in any adult insect. The middle thoracic segment may carry a pair of wings or wing-covers, and the third segment a pair of wings. [8] In some Insects there are traces of a fourth thoracic segment.