STRUCTURE OF PERIPATUS 



363 



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 oeso- 

 phageal ring with the two widely separate latero-ventral nerve-cords. 

 These are connected transversely by numerous commissures, are shghtly 

 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 



sl.g. 



oe.co 



Fig. 203. — Dissection of Peripatus. — After Balfour. 



at., Antennas ; or. p., oral papillae ; e.g., cerebral ganglia ; sl.d., duct 

 of slime gland {sl.g.) ; s.o.S, eighth segmental organ or nephridium ; 

 V.C., ventral nerve connected by transverse commissures (co.) with 

 its fellow ; s.0.17, seventeenth nephridium ; g.o., genital aperture ; 

 A., anus; p.d.c, posterior commissure; F./j, seventeenth 

 appendage ; a.g., last crural gland — that of the opposite side is 

 marked v.g. ; F./, F.2, first and second legs ; oe.co., oesophageal 

 nerve commissure ; oe., oesophagus ; ph., pharynx — the remainder 

 of the gut is removed. 



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 saUvary glands, which extend far back along the body. 

 Mouth, pharynx, and short oesophagus are lined by a chitinous cuticle, 

 hke 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 " haemocoele." 



