168 ARTHUR E, SHIPLEY. 



ectodermal cells which have kept their ordinary character, and 

 the inner wall is composed of a few cells of enormous size, 

 which all but obliterate the cavity of the cup. These cells are 

 wedge-shaped, and their broader ends are crowded with small 

 spherical concretions, which do not dissolve in alcohol, 

 chloroform, or benzine. Towards their outer, narrower ends 

 these cells become free from these concretions and stain 

 uniformly, or else they contain from two to five or six large 

 star-shaped aggregations of crystals (fig. 9). Nothing like 

 these were seen in the papillae of Ph. varians. 



The papillae on the introvert stand out much farther from 

 the level of the skin than those of the trunk (fig. 2). They are 

 conical in shape, with a narrow basis. At their apex is a pore, 

 and microscopic sections show that this is surrounded by a 

 number of minute horny plates similar to those in the papillae 

 of the trunk, though much smaller, and not visible like the 

 latter on the surface view. No crystals were found in 

 the papillae of the introvert, but the large wedge-shaped cells 

 ■were in other respects similar to those described in the trunk 

 papillae. 



The Head. 



The head is represented diagrammatically in fig. 5. Part 

 of the outer limb of the double horseshoe-shaped lophophore, 

 involving eight or ten tentacles, has been cut out in order to 

 show the pigmented region in the hollow of the original lopho- 

 phore, and to display more clearly the arrangement of the 

 tentacles. The thin transparent collar is extended over the 

 head, a condition in which it is usually found when the intro- 

 vert is retracted. The crescentiform mouth is shown sur- 

 rounded by the lower lip, and at the base of the pigmented re- 

 gion on the dorsal side lies the brain, directly continuous at 

 each side with the epidermis. 



The tentacles are very numerous, seventy to ninety. Like 

 those of Phymosoma varians, they are roughly triangular 

 in section. One side, that directed towards the mouth, is 

 grooved, and the groove is lined with cilia; the grooves of the 

 various tentacles tend to fuse together near their lower ends. 



