l6 EISEN 



thelial cells lining the atrium, or the ducts may enter directly between 

 the cells of the atrium. In other species, again, these ducts run all 

 the way down to the pore of the penis and open there between the 

 epithelial cells, or they may continue to the very pore, opening onto 

 the free surface around the pore, still remaining inside the sheath of 

 the sperm-duct. In some instances the ducts of these glands spread 

 out between the epithelium and the muscular layers of the atrium and 

 form a thick layer of irregularly running threads. Some of these 

 narrow ducts run upwards in the atrium, while others run downwards 

 to the pore some little distance before they finally penetrate the epi- 

 thelium of the atrium in order to empty their contents in the atrial 

 lumen. Through this arrangement nearly the whole anterior surface 

 of the atrial lumen is evenly lubricated by the secretions of the glands 

 and clogging at any given point is most effectually prevented. The 

 individual ducts of the glands are so rninute that they may be readily 

 mistaken for fibers. The lumen of the duct is not demonstrable by 

 present microscopical means and the nature of the duct can only be 

 judged by following some of the ducts until they empty their content 

 in the atrial chamber. The great variety of arrangement of these 

 glands is illustrated in the various figures. 



Accessory glands. — As ' accessory glands ' I have referred to glands 

 which open around the penial bulb and which do not enter this bulb. 

 In structure the accessory glands resemble the atrial glands, and like 

 them are composed of fascicles of unicellular glands, the ducts of which 

 never fuse. Accessory glands are comparatively rare. So far they are 

 found in only a few species, such as M. pedatus^ JSI. solifitgus., M. 

 fontinalis ., and M. franciscanus. In the latter species there is 

 only one accessory gland, but this one is of enormous size (p1. iv, 

 fig. 4) . 



Penial glands. — As ' penial glands ' I refer to all glands which are 

 confined to the penial bulb. They are of at least three distinct kinds, 

 according as they open into the sperm-duct, into the penis, or simply 

 around the penial pore. The majority of the penial glands open 

 around the pore outside of the sperm-duct. Other smaller glands 

 penetrate the sperm-duct from the exterior, while other glands are 

 entirely confined to the interior of the sperm-duct. Of the latter we 

 have examples in M. asiatlcus^ M. maculahis ., M. grandis^ and J/. 

 beringensis . 



Any of the above-mentioned glands may be present or absent. Very 

 few species possess all the various kinds, and in but one species, so 

 far as now known, are they all absent. The presence or absence of 



