ALEXANDROMENIA AGASSIZI. 135 



several folds arise and posteriorly extend along the groove to the cloacal cavity. 

 These are not constant for at their first appearance they are eleven in number 

 (Plate 20, fig. 9), soon decreasing to nine and gradually to five in the posterior 

 half of the animal. Each is penetrated by a loose meshwork of muscle and con- 

 nective-tissue fibres, through which the multitudinous ductules of the pedal 

 gland take their course, together with many corpuscles from the overlying sinuses. 

 The component cells are high and columnar, especially the outermost which con- 

 tain small (luantities of yellow pigment. 



The opening into the atrium, holding its customary subterminal ])osition 

 (Plate 7, fig. 3), leads into the atrial cavity whose walls are differentiated into the 

 usual ridges (Mundleisten) and cirrose area. As in P. hmvaiiensis the outer- 

 most ridge is accompanied throughout its anterior half by a prominence, ill 

 defined, and yet evidently sensory since it is composed of slender cells with 

 basal nuclei resting upon a rod-like group of ganglion cells. In the posterior 

 half of the lips this structure becomes more indistinct and finallj' blends indis- 

 tinguishably with the outer atrial ridge. 



Of the two large atrial ridges the outer is of large size and skirts the cavity 

 save on its posterior face. It is supported by an abundance of connective- 

 tissue fibres associated with a scant amount of muscle bands among which small 

 blood sinuses make their way. The halves of the dorsal ridge arise independently 

 of each other in the mid line on the roof of the atrial cavity. At first very low 

 they rapidl}^ increase in height (Plate 20, fig. 1), but behind gradually disappear 

 in the neighborhood of the opening into the pharynx. Blood sinuses penetrate 

 into their interior and probably in life increase these organs to a very considerable 

 degree. On each side of the mid line in front and hanging from the roof of the 

 cavity are two pairs of large papillae springing from the outer and inner ridges 

 respectively. The epithelial covering is composed of columnar cells whose 

 distal half is filled with golden yellow pigment. 



Each cirrus arises independently as a finger-shaped process of the atrial 

 wall with an average length of 0.3 mm. It is composed of cells about twice as 

 high as wide, closely packed with yellow pigment, arranged about a central 

 cavity within which it is occasionally possible to follow a nerve fibre. 



The opening of the mouth into the pharynx is guarded by a circular fold 

 beyond which the canal passes dorsally until in the neighborhood of the radula 

 where it bends at right angles and passes directly backward to join the stomach- 

 intestine. Throughout its entire extent its internal lining is developed into many 

 folds, large and small, often exceedingly wavy and of most complicated appear- 



