THE CTENOPHORA 



careous sphaeroids (otoliths) ; and, according to Samassa (21), 

 when the otoliths are fully formed, they are ejecte'd, still sur- 

 rounded by the remnants of the cells in which they were formed, 

 and become aggregated together to form a mulberry-like mass. 

 The otolith mass is supported by four " balancers," delicate lamellae 

 of peculiar shape formed by fused cilia. The whole structure, 

 sensory pit and otolith mass, is covered over and protected 

 by a transparent dome formed by fused cilia (see Fig. II. 1, 2). 

 The four balancers lie in the angles of intersection of the sagittal and 



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FIG. II. 



1. Surface view of the sense organ of Hormiphora plumosa. Pf, polar fields ; a, ampullae ; 

 xp, excretory pores ; x, groups of gland cells ; c/, ciliated furrows. (After Chun.) 

 2. The same seen from the side, ot, otolith mass ; <M, cupule formed of fused cilia. 



transverse planes ; from the base of each of them two rows of ciliated 

 furrows run outwards to end in the uppermost comb of each costa. 

 The sensory pit is produced on either side, in the sagittal plane, 

 into an elongated band -like ciliated tract. These tracts are 

 known as the polar fields (Fig. I. 4, and Fig. II. 1, Pf), and it 

 was supposed that they served as olfactory organs, but Samassa 

 (21) states that they are nothing more than tracts of simple 

 ciliated epithelium, devoid of sensory cells, so their function re- 

 mains unknown. Samassa denies the existence of any nervous 

 structures beyond those already mentioned; but Hertwig (13), 

 whose observations have recently been confirmed by Bethe (5), 

 describes a sub-epithelial nerve plexus similar to that which occurs 

 in Medusae. 



