ANATOMY AND DEVELOPMENT OF PERIPATUS OAPENSIS. 243 



cavity of the feet in various directions (PL XVIII, fig. 20). 

 There is no special circular layer of fibres. 



Histology of the muscle. — The main muscles of the 

 Dody are unstriated and divided into fibres, each invested by a 

 delicate membrane. Between the membrane and muscle are 

 scattered nuclei, which are never found inside the muscle 

 fibres. The muscles attached to the jaws form an exception 

 in that they are distinctly transversely striated. 



The Body Cavity and Vascular System. 



The Body Cavity, as already indicated, is formed of three 

 compartments — one central and two lateral. The former is by 

 far the largest, and contains the alimentary tract, the genera- 

 tive organs, and the mucous glands. It is lined by a delicate 

 endothelial layer, and is not divided into compartments nor 

 traversed by muscular fibres. 



The lateral divisions are much smaller than the central, and 

 are shut off from it by the inner transverse band of muscles. 

 They are almost entirely filled with the nerve-cord and salivary 

 gland in front and with the nerve-cord alone behind, and their 

 lumen is broken up by muscular bands. They further contain 

 the segmental organs which open into them. They are pro- 

 longed into the feet, as is the embryonic body cavity of most 

 Arthropoda. 



The Vascular System is usually stated to consist of a dorsal 

 heart. I find between the dorsal bands of longitudinal 

 fibres a vessel in a space shut off from the body cavity by a con- 

 tinuation of the endothelial lining of the latter (fig. 16). The 

 vessel has definite walls and an endothelial lining, but I could 

 not make out whether the walls were muscular. The ventral 

 part of it is surrounded by a peculiar cellular tissue, probably, as 

 suggested by Moseley, equivalent to the fat bodies of insects. 

 It is continued from close to the hind end of the body to the 

 head, and is at its maximum behind. In addition to this vessel 

 there is present a very delicate ventral vessel, by no means 

 easy to see, situated between the cutis and the outer layer of 

 circular muscles. 



