240 MARTIN r. WOODWARD. 



The venous system takes tlie form of series of more or less 

 well-marked canals and sinuses, which are specially conspicu- 

 ous in the region of the right kidney. The blood from the 

 foot and anterior parts of the body is apparently collected 

 into a series of channels, which run in close connection with 

 the excretory epithelium of the anterior lobe of the kidney, 

 while that from the liver and stomach passes through the 

 posterior lobe. These various renal veins eventually open 

 into a large sinus situated ventrally to the ureter, genital 

 duct, pericardium, and rectum (figs. 7 and 23, v. s.), from 

 which the afferent branchial vessels arise. 



The body-cavity of the adult is very inconspicuous, owing 

 to the great development of the crop with its radiating 

 muscle-fibres. This cavity represents part of the venous 

 system, and is of the nature of a bsemocoele. The true coelom is 

 only represented in the pericardial, renal, and genital cavities. 



The Nervous System. — An examination of fig. 27 will 

 show at a glance that the nervous system of P. Beyrichii 

 presents all the essential features of that of a typical Dioto- 

 cardian, this being especially noticeable in the practical 

 absence of distinct ganglia ; for although on the removal of 

 the dense connective-tissue sheath a certain amount of orange 

 colour is noticeable in the cerebral and pedal centres, thus 

 indicating an accumulation of nerve-cells, yet an examination 

 of a series of sections through these regions and the inter- 

 vening connectives shows (fig. 22) that while the nerve-cells 

 are more abundant in these coloured areas, yet they are not 

 confined to these regions, but are distributed, though in 

 smaller numbers, throughout the whole length of the connec- 

 tives, commissures, and even many of the nerves. The 

 orange-coloured areas where the nerve-cells are more abun- 

 dant correspond with the cerebral centres (c6. g.), the points 

 of origin of the visceral loop {pi. c), and the anterior portion 

 of the pleui'o-pedal cords. This distribution of the nerve- 

 cells along the connectives makes it extremely difiicult to 

 localise the individual ganglia, and forces us to rely rather 

 upon the points of origin of certain nerves than upon the 



