398 _-E. W. MACBRIDE. 
pletely with the skin in front, Professor Spengel concludes 
that the dorsal chamber is a forward diverticulum of the trunk 
ceelom behind the branchio-genital region. 
The csophagus is constricted into two tubes—a 
brauchial tube above and a true esophagus below— 
which are separated from each other by the apposition 
of two thickened ridges (fig. 14 d). The intestinal region 
of gut (behind the liver region) shows a lateral paired 
or unpaired groove with strong cilia. There is an 
external circular layer of muscles throughout the 
whole trunk, but the circular muscles of the proboscis are 
very thin. 
The peripharyngeal cavities are of great extent, 
meeting each other inthe mid-dorsal and mid-ventral 
lines; and since it is in the anterior walls of these cavities 
that the efferent vessels of the glomerulus run, it follows that 
in this genus they have a vertical course, and unite with each 
other to form aventral collar plexus before opening 
into the ventral vessel of the trunk. 
-The central nervous system is connected with the dorsal 
epidermis by several dorsal roots. These latter have a 
sheath of nerve-fibrils, and hence there is a mantle of white 
matter on the dorsal as well as on the ventral and lateral 
surfaces of the cord. This genus contains nine species; the 
peculiarities of the more minutely investigated are mentioned 
below. 
Pt. minuta (Plate 30, fig. 2) has very feeble genital ridges, 
into which the gonads send only rudimentary projections ; 
8—9 cm. long. Its collar skeleton is shown in fig. 8, and its 
central nervous system has the structure described in the 
introduction. 
Pt. sarniensis, very similar, but much larger, about 50 cm. 
long. Central nervous system traversed by a single canal 
throughout, which, however, does not open into the posterior 
ectodermic sac. 
Pt. aperta has great genital wings starting from imme- 
diately behind the collar, but not approaching each other; the 
