320 PHYLUM ARTHROPODA. 
Skin.—The chitinous cuticle, ordinarily thick in Arthropods, is 
delicate. It is subject to moulting. -The epidermis is a single layer of 
cells. Beneath it there is a dermis. i 
Muscular system.—Externally there is a layer of circular muscles ; 
within this lies a double layer of diagonal fibres ; internally there are 
strong longitudinal bundles. Finally, in connection with this internal 
layer, there are fibres which divide the apparent body cavity into a 
median and two lateral compartments, The median includes heart, 
gut, slime glands, reproductive organs; the laterals include the nerve- 
cords and salivary glands; the legs contain nephridia and coxal or 
crural vesicles. Striped, rapidly contracting muscles are characteristic 
of Arthropods, but in Perdpatus the muscles are unstriped, excepting 
those which work the jaws and are perhaps the most active. The true 
ccelom is represented in the embryo by the cavities of the mesoderm 
segments, which give origin to the muscular system. 
Nervous system.—The dorsal brain is connected by an ceso- 
phageal ring with the two- widely separate latero-ventral nerve-cords. 
These are connected transversely by numerous commissures, are slightly 
swollen opposite each pair of legs, to which they give off nerves, and 
are united posteriorly over the anus. There are only hints of ganglia, 
but there is a continuous layer of ganglionic cells. The brain is very 
homogeneous, simpler than that of most Insects. Sense organs are 
represented by two simple eyes on the top of the head. These are 
most like the eyes of some marine Annelids. 
Alimentary canal.—Round about the mouth papillae seem to 
have fused to form a ‘‘ mouth cavity,” which includes the mandibles, « 
median pad or tongue, and the opening of the mouth proper. The 
mouth leads into a muscular pharynx, into which opens the common 
duct of two large salivary glands, which extend far back along the body. 
Mouth, pharynx, and short cesophagus are lined by a chitinous cuticle, 
like that of the exterior. The long endodermic digestive region or mid- 
gut extends from the second leg nearly to the end of the body. . Its 
walls are plaited. Finally, there is a short rectum or proctodeum, 
lined by a chitinous cuticle. 
Circulatory system.—The dorsal blood vessel forms a long con- 
tractile heart. It lies within a pericardial space, and receives blood 
by segmentally arranged apertures with valves. The circulation is 
mostly in ill-defined spaces in the apparent body cavity or ‘*hzmo- 
coele.” 
Respiratory system.—Very long and fine unbranched trachez are 
widely distributed in the body ; a number open together to the exterior 
in flask-like depressions. These openings or stigmata are irregularly, 
distributed. 
Excretory system.—aA pair of nephridia lie in each segment. 
Each consists of an internal mesodermic terminal funnel, a looped canal, 
and a wide vesicle which opens near the base of each leg, the two last 
parts being invaginations of the ectoderm. Nephridia are not known in 
any other Tracheate. The salivary glands and the genital ducts seem to 
be modified nephridia. It may be noted, too, that the same is perhaps. 
true of the ‘‘coxal glands” of ZLzwewlus and of the antennary glands 
of Crustaceans, 
