PRONEOMENIA ACUMINATA. 217 



shaped cirri, is in reality a modified snout homologous with that in the Chitons, 

 and perhaps with the sensory shield in the Chaetodermatidae. On such an 

 assumption the true mouth is deeply seated, and in the present case is in the 

 neighborhood of the outlet of the dorsal salivary gland. 



In the specimen in hand the atrial opening, subterminal in position is a 

 relatively long narrow slit leading into a spacious chamber, the atrial cavity, 

 whose hmits are fairly well-defined by two prominent horseshoe-shaped ridges, 

 which fuse posteriorly (Plate 5, fig. 1). The first of these, the outermost and 

 continuous across the mid line, courses parallel to the borders of the outer 

 opening, and is bounded externally by slender cells bearing a well-defined coat 

 of cilia. Internally it is supported by an abundance of connective tissue and 

 is penetrated by a blood sinus distended by blood corpuscles. It thus appears, 

 as has been suggested, that these organs may play a part in the process of respira- 

 tion, but the nature of the overlying epitheUum and the presence of nerve 

 fibres beneath it indicates also that it is a sense organ though its office is unknown. 



As in Proneomenia hawaiiensis and some other neomenians, this external 

 atrial ridge is bounded on its outer side by a lower prominence (Plate 5, fig. 1) 

 whose slender cells are fully twice as high and are evidently sensory. In this 

 species fibres may be traced into its neighborhood, but it is not so evident that 

 they are sensory as in a few other species. In P. hawaiiensis a cord-hke group 

 of ganglion cells is situated in close proximity to the overlying ridge, which 

 it supphes with numerous deUcate fibres, and in the opposite direction is united 

 at fairly frequent intervals with some of the gangUa located about the bases 

 of the cirri. There thus appears to be little doubt that in such cases we are 

 deaUng with a definite sense organ, and it is probable that in Proneomenia acumi- 

 nata the same is true. 



Throughout its entire extent this external sensory ridge contains a small 

 number of gland cells, whose slightly vacuolated secretion stains intensely in 

 haematoxylin. In the posterior third of its course additional cells of the same 

 character appear in the outlying hypodermis beneath the spiculose invest- 

 ment, and in this position they continue to the posterior border of the atrial 

 cavity. 



The inner atrial ridge, in form and structure and perhaps in function, 

 is the counterpart of the external one (Plate 5, fig. 1). 



In the area circumscribed by these prominences the cirrose area is located, 

 and is characterized in the present instance by numerous slender diverticula 

 of the atrial wall. These arise singly, and are further distinguished by multi- 



