152 HKUrOMKNIA PLATVFODA. 



and varied in size fmni 1 1 nun. in Icnglli and (I.O nnn. diameter to 18 mm. long 

 and O.U mm. in thickne.ss. A well-defined dorsal keel extends throughout 

 the entire length of the animal with the exception of the extreme pos- 

 terior end. In a eontraeted state both ends are pointed and quite similar, 

 but usually tlie atrial cavity is opened so that the outer ridges are exposed, thus 

 giving the front end a blunt appearance. The color is white or yellowish white, 

 depending to considerable degree upon the food contained in the alimentary canal. 

 A single layer of spicules envelops the body, the majority being leaf-like in form 

 and biconvex in cross section. A second type with a short stem occurs in 

 the neighborhood of the ventral furrow, and to a less extent over the body 

 generally. The cuticle is more highly developed than is usual with species in 

 this family, and is underlaid by a hypodermal layer of more than usual height 

 beneath the dorsal keel. In their general appearance the component cells 

 resemble those of some of the Chaetodermatidae, being of cubical or rectangular 

 shape, or in the case of what appear to be sjiicule matrix cells more or less 

 globular except in the region of the keel where they are much elongated. 



The anterior pedal gland (Plate 19) occupies practically all of the head region 

 not held by the ventral salivary glands from slightly in front of the middle of the 

 pharynx to the forward end of the body. The crypt into which it opens is typi- 

 cally placed (Plate 8, fig. 1), possesses unfolded walls and is profusely ciliated. 

 Posteriorly the anterior gland joins, without any sharp line of demarcation, the 

 posterior one which continues to the cloaca. 



In the anterior end of the body the foot is the merest fold or it may be en- 

 tirely smoothed out. This latter condition obtains in the posterior half of the 

 body (Plate 19, tig. 10). However, all of the cells retain their ciliated condition 

 though they are more colunmai' than the ordinary pedal cells of other species. 



The atrial cavity is unusually small (Plate 8, tig. 1) and the outer ridges are 

 lacking or are without clearly defined boundaries. The inner elevation on the 

 other hand is a prominent fold, penetrated by blood sinuses, and is composed of 

 cubical cells save along the free border where they are higher, more sjwngy 

 and heavily ciliated. The cirri, presenting the customary appearance, are ar- 

 ranged in groups of 3-7. The opening from the atrium into the buccal-pharyn- 

 geal section is comparatively narrow, being reduced by a large fold springing 

 chiefly from the dorsal side of the digestive tract. Beyond this the canal widens. 

 Its hnmg epithelium becomes considerably folded, and is composed of columnar 

 cells endowed with considerable glandular activity. Still farther inward the wall 

 again develops a circular fold which forms the outer boundary of another circu- 



