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. 



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 Peripatus 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, a 

 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 oesophagus 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 proctodaeum, 

 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 "haemo- 

 ccele." 



Respiratory system. Very long and fine unbranched tracheae 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. A pair oi 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 Ltmulus and of the antennary glands 

 of Crustaceans. 



