BY JAS. P. HILL. 
27 
by a layer of columnar cells with long cilia just as in the described 
species, and the "tongue floor" composed of cells poor in proto- 
plasm is also, as in them, infolded into the cavity of the "tongue." 
This cavity (fig. 21, gtc.) just as in Pt. minuta is lined by an 
irregular peritoneal layer, and also contains fibres which stretch 
across between its opposite walls at a short distance below the 
"tongue back." 
The gill pores (fig. 20, gp.) are narrow slits slightly obliquely 
placed, which open into the longitudinal grooves bounding the 
"gill area" laterally, and as in the described species the longi- 
tudinal muscles lying laterally to the pores give off fine bundles 
which pass between adjacent pores to join the longitudinal 
muscles on the inner side of the gill pores. 
The anterior ends of the gill rows project for some distance 
into the posterior region of the throat (fig. 18, ay.), but owing to 
the oblique direction of the most anterior gills the posterior rim 
of the collar forming the so-called "operculum''" of Bateson covers 
at most only the first gill pore. The conditions are essentially 
the same as in Pt. clavigera since the genital wings do not possess 
free anterior edges, but are united with the hind edges of the 
collar, thus enclosing between them and the "gill area" the space 
for which Spengel adopts the term "atrium." It is into the 
narrow anterior projection of this space below the posterior edge 
of the collar that the first gill pore opens. 
The number of gills varies according to the age of the animal, 
since new ones are being continually formed during life at the 
hind end of the gill region. As in Pt. minuta the number of 
gills never appears to exceed 40 pairs, but in the majority of 
individuals the number is very much less than this. The gill gut 
is not closed blindly behind as in Pt. minuta, but passes directly 
over into the gut of the succeeding genital region. 
(2) P o st-b r an c hi a I r e g io n. — Genital region : The mus- 
culature and the dorsal and ventral vessel stems are essentially 
related as in the branchial region. The wall of the gut (fig. 23, /.) 
in this region is usually folded, and lies at a varying distance, 
usually small, from the longitudinal musculature, and there pass 
