400 HAROLD HEATH, 



absolute alcohol and all grades above 80 "/<, were not satisfactory ; 

 Flemming's fluid gave excellent fixation but darkened the tissues badly; 

 while Vom Rath's solution was in many respects the most satisfactory, 

 especially when followed with a 1 ^/o solution of pyroligneous acid. 

 Specimens thus treated were especially good for the study of the 

 nervous system. 



Delafield's haematoxylin and Hetdenhain's iron haematoxylin 

 with a light secondary stain of rubin were, so far as my experiments 

 show, among the better stains. In Proneomenia especially, many 

 nerves, such as some of the pediil and palliai commissures, traverse 

 the visceral cavity for relatively long distances and are not firmly held 

 by connective tissue or muscle fibres. Hence to prevent their dis- 

 lodgment it is necessary to exercise extreme care in fixing sections 

 to the slide and in their subsequent treatment. 



The genera concerned in the present paper are a single species 

 of Proneomenia and Pliopalomenia from the Hawaiian Islands. In 

 every case the results discussed in the following pages have been 

 gained from dissections and have been confirmed by a careful study 

 of sections. 



In the species of Proneomenia under discussion the mouth opening, 

 subterminal in position, and having the form of an elongated slit, 

 leads in to a buccal cavity that in fully expanded specimens is of 

 generous proportions (Fig. 4). As in other members of the genus the 

 inner surface of the buccal wall is richly supplied with sense organs. 

 The most conspicuous of these consists of two horseshoe-shaped ridges, 

 composed of high columnar cells, richly ciliated and containing relatively 

 large blood sinuses. The first projection is located a short distance 

 within the mouth cavity which it entirely surrounds save on the 

 posterior wall. The other is situated close to the junction of the 

 mouth and pharynx though its hinder free extremities unite with the 

 corresponding portions of the external one. In other words these two 

 ridges are inclined to each other at an angle of about 30° thus em- 

 bracing an area that may be termed the cirrose region, which is 

 characterized by the presence of numerous inwardly projecting finger- 

 shaped processes that function as sense organs or possibly collectors 

 of food. Another sensory prominence of low elevation is located ex- 

 ternal to the outer of these larger ridges with which it is coextensive. 

 Throughout its entire extent it is in immediate contact with numbers 

 of ganglionic cells and its constituent elements are almost altogether 

 sensory cells. 



