BY JAS. P. HILL. 27 



)>y 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 cavit3^of the "tongue." 

 This cavit)^ (fig. 21, ytc.) 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 jDores (fig. 20, gj).) are narrow slits slightly obliquel}'^ 

 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 lietween adjacent pores to join the longitudinal 

 muscles on the inner side of the gill pores. 



The aijterior ends of the gill rows project for some distance 

 into the posterior region of the throat (fig. 18, ay.), but owing to 

 the ol^lique 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 ]H. clarigera since the genital wings do not possess 

 free anterior edges, Ijut 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 ^'erv much less than this. The sill out 

 is not closed blindly behind as in Pt. minuta, but passes directly 

 over into the gut of the succeeding genital region. 



(2) P OS t-b r anchial r eg 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 



