14 T. KOMAI : STUDIES ON TWO AUICRRANT CTENOPHORES 



not always, thinner than the dorsal epidermis, measuring 18-25^ in the 

 peripheral parts of the body, and somewhat less in the central. In 

 regions neighbouring the mouth aperture, the thickness of the ventral 

 epidermis scarcely exceeds 10//. The epidermis is thinnest in parts 

 directly under the tentacle-sheath, where it is as thin as 5 // on the 

 average. 



There exists nothing like a basement membrane under the epider- 

 mis, although its presence is maintained by Abbott ('07). 



Aboral S ens e- o rga n. 



The aboral sense-organ is situated at the centre of the dorsal 

 surface of the body directly above the mouth-aperture. In living specimens, 

 the situation of the organ is indicated by a small roundish unpigmented 

 area readily recognized from the surrounding parts. In the surface view 

 under the microscope (PI. 4, fig. 6), the organ presents itself as a deep 

 depression somewhat elongate in the transverse direction and containing 

 an otolithic mass in the centre. On either sagittal side of the entrance 

 into the cavity of the depression, there is an area shaped semilunar and 

 covered with cilia, apparently to be identified as the polar plate (/. /). 

 In minute points in the structure of this part, C bocki differs somewhat 

 from the other two species, C willeyi and C. mitsukuni. In C. bocki, 

 the part is raised into a rather tall crest-like prominence, of which the 

 inner side is formed by ciliated epithelium and the outer by ordinary 

 epidermis. Evidently, the entire ciliated part represents the polar plate. 

 The free margin of the prominence is generally divided into from two 

 to five lobes of various sizes (figs. 6, 7), although it is by no means rare, 

 especially in small specimens, that the margin is quite even and lacks 

 lobation entirely: the prominences of the two sides seldom keep up a 

 strict symmetry. In C. tvilleyi and C. mitsukurii, the lobation of the 

 polar plates does not occur even in the largest individuals, the entire 

 area being of a simple semilunar shape. 



As in ordinary ctenophores, the polar plate consists of a central 

 area {c. a) and a marginal area encircling the former (m. a) (" Polfeld " 

 or " Mittelfeld " and " Randwulst " of previous writers; Chun, '80, 

 Hertwig, '80). The central area is made up of a flat epithelium, while 

 the marginal area is formed of a thicker one. Through the central area 

 appears a number of circular muscle-fibres which functions as the 

 sphincter of the sensory cavity. The peripheral part of the marginal 



