98 DOIJYMENIA ACUTA. 



nerve cells (•(imiect by I'elatively large nerve bundles with the ganglia located 

 near the bases of the cirri. 



Of the ciliated I'idges (Mundleisten) the more external are in the form of 

 two elevations wliich approach each other very closely in front and behind, 

 at which ])oints they become low and inconspicuous though in their mid section 

 they are comparatively high. The inner ridge has the form of a horseshoe, the 

 free extremities connecting posteriorly with the ends of the outer ridge. 



This inner prominence is relatively short yet high, and like the outer con- 

 tains a loose mesh work of muscle and connective-tissue fibres among which are 

 numerous lilood coriniscles. The cells composing them present much the same 

 appearance as those of the outer low elevation described in the preceding 

 jiaragraph. Practically all are slender and contain small amounts of pigment 

 and elongated nuclei. Nerve fibres may be followed into the ridges which thus 

 seem to be sensory. In P. hawaiiensis these cells are richly ciliated, but in 

 this species all traces of cilia are absent and, it may be added, the material is 

 excellently jireserved. 



Williin the cirrose area and lying behind the innermost ridge the atrial 

 wall in the mid line is develojied into a fold, of large size, which is closely packed 

 with multitudes of l)lood corpuscles. If the buccal ridges serve as respiratory 

 organs, as some authors would have us believe, this structure is certainly more 

 efficient since it is not only voluminous but its epithelial covering is not more 

 than one third as thick as that of the general atrial cavity. 



The cirri are prominent structures in this species, being not only numerous 

 but of considerable length and calibre. Each is composed of cubical or low 

 cohunnar cells filled to a considerable extent with the usual yellowish pigment 

 which more or less conceals the small centrally placed spherical nucleus. At 

 various points these organs may arise singly from the buccal wall, but usually 

 the bases of from four to six are fused, and into this stalk muscle and occasionally 

 nerve fibres may be traced. The cavity of the single cirrus is usually so small 

 that the relation of these fibres remains unknown and, it may 1)6 noted, efi'ec- 

 tually blocks the entrance of blood cells, so that these organs are rather to be 

 considered retractile sensory organs with little respiratory function. 



A shoit distance behind the cirrose area the pharynx originates as a tube 

 with somewhat smaller diameter than the atrial cavity. However, immediately 

 behind the region of the brain the canal from external view expands considerably, 

 but sections of this region show that a great fold develops in the pharyngeal 

 wall which it entirely encircles reducing the cavity to'a crescentic slit (Plate 15, 



