The head properly so called, namely the circular shield-like plate seen upon the dorsal surface at the anterior extremity of the body, although apparently consisting of a simple horny disc, is doubtless composed of several segments conjoined superiorly ; indeed, these are completely confused, and inferiorly are too soft and membranous to be distinguished, except by the presence of those articulated appendages, which, although forming parts of the mouth, are still merely repetitions of the jointed legs affixed to the other segments of the • body. Thus the most superficial plate, with its articu lated appendage, the labium and labial palpus of entomologists, is but an incomplete ventral scutum, with its articulated limb in a rudimen tary condition' as compared with those of the body, and is even armed with a distinct claw, as are the locomotive legs. In like manner the second pair, the maxilla of authors, are legs but one step further removed from their normal form, but not more so than are the poison fangs already described. In the third pair or mandibles we have a leg reduced to its terminal claw, and that is broad and serrated so as to become useful in manducation. Lastly, the corneous and serrated piece (the labrum) seems to be the last vestige left of limbs of this description, the two horny remnants of legs having become consolidated with each other and with the dorsal head-plate, so as to form the anterior boundary of the mouth.
Alimentary canal.—The alimentary canal in all the Myriapoda is of extremely simple con struction, and both in its form and general arrangement, very nearly resembles that of the larva: of Lepidopterous Insects. In Julus ter restris (fig. 311) the esophagus (Ii) is seen to be of considerable capacity, in accordance with the nature of the coarse food upon which these vegetable-eating species live. The stomach is long and bowel-like, extending from the termi nation of the esophagus to the insertion of the hepatic vessels. To this succeeds a wide and sacculated colon, which passes directly to the anal segment of the body, where it terminates.
In the Scolopendroid genera the same con formation of the alimentary apparatus is met with, the stomach and intestine passing straight from the mouth to the anus without any pecu liarities of structure worthy of notice.
In Lithobius finficatns, which we may take as a specimen of one of the Chilopod Myria )oda, a very similar arrangement exists; the oesophagus, which is pro portionately narrow, ends m a simple stomachal enlargement of an oblong shape, and this termi nates in a straight bowel, the point of separation be tween the one and the other being only indicated by the entrance of the biliary vessels.
The glands connected with the alimentary ap paratus closely resemble those of the insect larva. Two convoluted salivary tubes are seen folded tip at the sides of the oeso phagus, where their con volutions are intervolved into a species of ravel (fig. 311, i) with the
origins of the hepatic ves sels (h), which latter, after a tortuous course, are in serted, as in Insects, at the termination of the sto machal portion of the di gestive tube.
Respiratory System.— The Myriapoda respire in the same manner as In sects by means of late ral spiracles and tracheal tubes. The spiracular ori fices are, in the Scolo pendridm, very conspi cuous, as, for example, in Lithobius, (fig. 312,) where the corneous lips of the apertures leading to the tracheae (s, s, s) are seen situated behind the origins of the legs, upon the sides of the 2d, 4th, 6th, 9th, 11th, 130, and 15th segments, occur ring upon the alternate segments, except in the case of the 86, where there is one missed. The trachea; derived from these spiracles pass inwards to he disbrilmted upon all the viscera, ramifying in every part of the body, and thus conveying air throughout the system. In structure these air-vessels exactly resemble those of true Insects, and are equally characterized by the existence of a spiral fibre in their interior, whereby they are always kept permeable.
Circulatory system—In the nature of their circulatory apparatus the Myriapoda are closely related to the Insects properly so called. A long dorsal vessel passes from the tail towards the head along the menial line of the body. The sides of this vessel, on clearing away the fat which surrounds it on 'all sides, are seen to be perforated at intervals with numerous valvular ori fices, through which the cir culating fluid gains free admis sion from the general cavity of the body, and by the un dulatory contractions of the dorsal heart thus constructed is forced forward toward the head. Arrived in the neigh bourhood of the oesophagus, the dorsal heart is seen to give off several vessels, and ac cording to the opinion of Mr.
Newport and Mr. Lord,* there is reason to suppose that a vascular system more com plete than has as yet been proved to exist in any of the true Insects may be pointed out in this region of the body.
The dorsal vessel itself, when examined under a microscope, is distinctly muscular, being formed of circular flat bands that surround the cavity of the tube, so that doubtless the action of this heart, in the larger species at least, is suffi ciently energetic.
Foramina repugnatoria.—These are a series of orifices which in the Julida are seen upon the lateral aspect of every segment of the body, and communicate with as many minute mem branous sacculi placed within the body. These sacculi, both from their position and relations, forcibly remind us of the series of respiratory sacs met with in the Leech and other air breathing Annelidans, but in Jutus they are supposed to be merely organs of secretion from which some offensive fluid is poured for the protection of the animal.