zgos.] excrp:tory organs of metazoa. 591 



33. XiPHOSURA (LiMULUS). 



Co.ral Glands. — A very thorough account has been given by 

 Patten and liazen (1900). The adult gland consists of four 

 nephric lobes at the bases of the second, third, fourth and fifth 

 legs, respectively, and these are connected medially by a stolon of 

 collective tubules ; the duct lies dorso-lateral from the latter, is much 

 convoluted and opens at the basis of the fifth leg. The duct arises 

 from a plate of cells of the somatic mesoblast of the fifth somite, 

 this plate invaginating to produce a funnel opening into a thin- 

 walled end sac that represents the fifth coelomic sac; the distal end 

 of the duct is formed by an ectoblastic invagination. Outgrowths 

 of the end sac finally unite with cell chains of adjacent nephric lobes. 

 In each of the six thoracic somites a mass of nephric cells arises 

 independently of the duct from the somatic mesoblast, and these 

 masses, of which the first and sixth ultimately disappear, form the 

 nephric lobes ; ofifshoots from the four persisting masses produce 

 the canals of the stolon. Thus there are in the embryo six pairs 

 of coxal glands, but only four of them persist in the adult. 



The genital ducts arise as deverticula of the opercular meso- 

 blastic sacs, and are to be compared with nephridia (Patten and 

 Hazen). 



34. Arachnida. 



(i) Araneida. 



Malpighian Vessels. — These are excessively dendritic and their 

 delicate end branches form a fine felt-work around the liver lobes ; 

 by a pair of main ducts these open into the intestine just anterior 

 to the rectal vesicle. They have been proved to be excretory 

 (Marchal, 1889, Bruntz, 1903). Balfour (1880) and Morin 

 (1888) described them as arising from the ectoblastic proctodseum; 

 but with the exception of Kishinouye (1890, 1894) who derived 

 them from the mesoblast, the other embryologists (Loman, 1887, 

 Schimkewitsch, 1897) find that they develop from the entoblastic 

 mid-gut. Locy (1886) described them as coming from the prester- 

 coral tube, but though the latter is probably entoblastic its origin 

 was not definitely settled. Renewed investigation is needed on this 

 question, but the entoblastic origin seems to be best authenticated. 



