366 PROCEEDINGS OF THE ACADEMY OF [-^iay, 



(a) — Gcnnarium very large, extending across the whole width of the 

 trunk, oesophageal tube present, immobile setae within the 



coronal cup, bucinedax (Forbes). 



(6) — Germariura small, rounded, no setae within the corona. 



(1) — -(Esophageal tube present, dorsal cuticula of trunk with trans- 

 verse thickened ridges, vorax (Leidy). 



(2) — CEsophageal tube absent, dorsal cuticula without such ridges, 



lentijormis Mecznikow. 



In the description which follows I shall refer for comparison mainly to 

 the description of Gast for lentijormis, since his account is by far the 

 most thorough. 



Anatomical, 



The external form is fairly well known. The trunk is somewhat 

 wider than high, rounded posteriorly, while anteriorly it is continued 

 as a large corona (Riissel, Mecznikow; cup, Leidy; net, Foulke; Muiid- 

 trichter, Gast). When fully extended the form is as shown in the 

 figures (PI. XVIII, 1-3), the corona very large and its aperture usually 

 in the horizontal plane but sometimes oblique. The posterior wall of 

 this aperture (mouth) may be rounded or notched, or may project 

 forward as a convex lobe. On the ventral surface of the trunk is the 

 rudimentary foot (Chitinring, Mecznikow; disk or sucker, Leidy; Fuss, 

 Gast) ; and behind that, also ventral, the cloacal aperture. The lat- 

 eral antennae are clearly visible on the sides. The whole animal is so 

 beautifully transparent that the greater part of the following descrip- 

 tion has been based upon a study of the living animal. 



Hypodermis and cuticula. — The hypodermis is a very thin layer, 

 with its flattened nuclei far apart; at the margins of the mouth it is 

 only slightly thickened. The cuticula is thin, colorless, and (especially 

 on the dorsal surface) covered with minute tubercles which may be 

 ovoid or stellate (as Gast found). On the antero-dorsal surface of the 

 trunk there are broad but narrow thickenings of the cuticula (Pl.XVUI, 

 X, fig. 1), the number and exact arrangement of which is subject to 

 considerable variation; each of these ridges bears tubercles. The 

 cuticula and hypodermis of the body wall are continued inward as the 

 lining of the corona; there the minute tubercles are replaced by minute 

 conical projections, borne mainly upon longitudinal ridges of the cuti- 

 cula. The whole cuticula is very flexible, and when the animal is 

 strongly contracted and the corona rolled inward it is thrown into 

 many folds; radial folds are also found around the cloacal aperture. 

 No cilia are found anywhere upon the surface of the body nor within 

 the corona, nor yet any immobile setae. 



