Fig. 29.—Heart of Julus terrestris showing ostia (ost) and arteries (Art) magnified.
Fig. 30.—Heart of Julus terrestris showing structure of artery (Art.) and external coat of heart (ext.c), also fat body (Fb), highly magnified. Ht, The cavity of the heart. The circular muscle fibres which surrounds the heart are shown just below the external coat (ext.c). ogl, Oil globules of the fat body.
Myriapods breathe by means of tracheae, with the exception of the Scutigeridae, which have an elementary form of lung which resembles that of spiders, and will be mentioned further on. These tracheae, as in Insects, are tubes lined with chitin, which is arranged in spiral bands. The tracheae open to the exterior by openings called stigmata, through which they receive the external air, which passes into the main tracheal tubes and into their ramifications, and thus effects the aeration of the blood.
The nervous system of the Myriapods consists, as in Insects, of a brain, which may be more or less developed, a circumoesophageal ring embracing the oesophagus, and a ventral chain of ganglia, and in some cases (Newport) of a system of visceral nerves. With the nervous system we may mention the sense organs, the eyes, which are present in most cases, though wanting, as has been already stated, in many groups. They are usually present as clusters of ocelli or eye spots closely packed together, or (in Scutigera) as peculiarly formed facetted eyes. The sensory hairs on the antennae must be reckoned as sense organs, as also the tufts of sense hairs on the head of Polyxenus. Scutigera has also a peculiar sense organ beneath the head, consisting of a sac opening on the under side of the head full of slender hairs, each of which is connected at its base with a nerve fibre. Except the eyes, the Myriapod sense organs have usually the form of hairs or groups of hairs connected with nerve fibres, which communicate with the central nervous system.
Fig. 31.—Under side of the head of Scutigera coleoptrata, with sense organ. eo, Opening of sense organ to the exterior; o, sense organ shown through the chitin; m, mouth; oc, eye; mxl, maxilla; f, furrow in the chitin. (Heathcote, Sense organ in Scutigera coleoptrata.)
Fig. 32.—Highly magnified section through head of Polyxenus lagurus, showing sense organ. ext.cut, external cuticle; t, tube surrounding base of sense hair; gang.c, ganglion cell. (Heathcote, Anatomy of Polyxenus lagurus.)
These two sense organs are shown in Figs. 31 and 32. Fig. 31 shows the under side of the head of Scutigera (Fig. 17), with the position of the sense organ and its opening. Fig. 32 is part of a section through the head of Polyxenus with two of the sense hairs. Each spine or sense hair fits into a cup in the chitin of the head; and the lower or internal part, which is divided from the upper or external part by a rim, is joined to a ganglionic nerve cell (gang.c.).