48 



EISEN 



Fig. 



Characteristics. — Externally this species is readily recognized by 

 the tawny color of its anterior somites, especially their dorsal part, 

 which color is caused by scattered granules of pigment. Internally the 



species is character- 

 ized by its six to eight 

 comparatively large at- 

 rial glands, which open 

 directly into the atrium 

 (fig. 23, a). 



Detailed descrip- 

 tion. 



Pigment . — The 

 granules of pigment 

 are found in both the 

 epithelial cells and in 

 the circular muscular 

 layer, but they are es- 

 pecially numerous in 



Mesenchytrceus fusctis. ^, ^ ^ c ^^ 



■^ ■' the outer part of the 



epithelial cells of the body-wall. Posterior to clitellum they are absent. 



Head-pore is situated about half way between the apex and somite I. 



Copzilatory organ. — As in many species oi Mesenckytrccus.,\h& 

 part of the sperm-duct nearest the male pore possesses two chambers 

 joining each other, the outer one being more properly a j)enis, while the 

 inner one is of a more glandular nature (fig. 22, a). In this inner 

 chamber and on the side nearest the intestine open the prostates. In the 

 specimens dissected and sectioned 

 there are some six to eight bvuiches 

 of these atrial glands, each opening 

 independently in the atrium. The 

 distal end of each glandular fascicle 

 is globula? or pear-shaped, while the 

 tubular end duct is narrow. This duct 

 is composed of a mass of tubes, which 

 jointly penetrate the atrium, forming a 

 thick layer of tubes between the mus- 

 cular and the glandular layers of the 

 atrium (p1. viii, fig. 5). The ducts of each fascicle surrounded by spi- 

 rally wound muscles, which seem to be mere outcroppings of the outer 

 muscular layer of the atrium. None of these glands open at the base 

 of the penis. The penial bulb consists of muscular strands arranged 



Fig. 23. MesenchytrcEus fuscus. 



