STRUCTURE AND DEVELOPMENT Ol'- COELOl'LANA. 15 



area is defined rather badly, the central area appearing to pass ahnost 

 directly over into the ordinary epidermis surrounding the polar plate. 

 The proximal part adjoining the sensory cavity, on the other hand, is 

 defined very distinctly, and is covered with heavy ciliation especially at 

 the median point of the free inner edge where exists a notch as a rule. 



The prominences about the polar plates are kept quite erect, and 

 the entrance into the sensory cavity is left wide open, as long as the 

 animal rests calm (fig. 6). But, when this is irritated by giving some 

 light stimulus, the free edges of the plates are suddenly brought together, 

 and close up the entrance, under which condition, the median notch just 

 mentioned becomes very prominent. If a little stronger stimulus be 

 given, the entire aboral region is retracted below the level of the general 

 surface of the body. The closure of the cavity is performed by the 

 sphincter fibres mentioned above, while the retraction of the entire aboral 

 region is worked out by collaboration of fibres abounding in the more 

 peripheral parts. The lobation of the polar plates in C. bocki reminds one 

 highly of the "sensory tentacles" described for Cteiioplana (Korotneff, 

 '86, WiLLEY, '96) and also of the papilliform processes known in Beroe 

 (Chun, '80), both of which occurring in the same part of the plate. 



The otolithic mass (fig. 6. of) is situated in the centre of the 

 sensory cavity and is seen doing the usual constant vibrating motion. It 

 is generally spheroidal, but may be of a somewhat irregular shape. 

 Frequently, there occur one or two supernumerary masses of small sizes 

 besides the large normal one. The granules composing each mass 

 (fig. 9) are globular, but may be of an irregular shape. Those of the 

 latter shape are apparently formed by conglomeration of two or more 

 globuler ones. 



From either transverse corner of the entrance into the sensory 

 cavity, there arise two fine ciliary tracts (fig. 6, cil. tr) each of which 

 passes along the interradial plane of the body. The tracts without doubt 

 represent similar ciliary tracts (" Nerven ", Chun, '80), that range between 

 the sensory capsule and the ribs in ordinary ctenophores. They can 

 be traced up to the cuter boundary of the unpigmented area, but no 

 farther. 



Under the polar plate shows through a pattern of a greenish hue 

 and of a crescentic shape or of a shape somewhat resembling the letter 

 W (fig. 6, g. g). This pattern is formed by an assemblage of peculiar 

 fine granules found in deeper parts of the capsular wall. Similar granules 

 are known in the same part of ordinary ctenophores (Chun, '2>6). 



