96 



COMPARATIVE ANATOMY 



CHAP. 



whole epithelium round this compound sensory organ, which is pro- 

 tected in special indentations in the disc margin by lobes, is a sensory 

 epithelium with a thick nerve plexus. There are either 4 or 8, less 

 frequently 12, 16, or even more rhopalia on the margin of the 

 Acraspede umbrella. 



III. The sensory body at the aboral pole, so characteristic of the 

 CtenopJwra, is a compound sensory organ of a very peculiar kind, which, 

 according to its structure, is an auditory organ, or ather, perhaps, an 

 organ for regulating the movement of the swimming plates. 



The sensory body is constructed as follows. It consists at first of 

 a shallow pit- like depression between the 4 branches of the aboral 



ec 



FIG. 73. Sensory bodies of Nausithoe, after Hertwig (optical transverse section of the margin of 

 the disc), sf, Sensory fold of the margin of the disc ; ec, ectoderm ; en, endoderm ; ga, gastro- 

 canal ; Jik, auditory body ; o, otolith ; se, sensory epithelium ; oc, eye ; I, lens ; g, jelly. 



so-called funnel vessel. The ciliated body epithelium which forms 

 the base of this pit thickens considerably (Fig. 75, se). Its elements 

 are thread-like cells. In this "sensory cushion" are found deposits 

 of pigment, which perhaps represent simple organs of sight. Eound 

 the edge of the pit there rises a membrane which unites above in the 

 shape of a bell, forming a sort of roof to the pit, which is thus trans- 

 formed into a vesicle. The membrane is composed of the long cilia 

 at the edge of the pit cemented together. It is broken through by 

 slits- at 6 places, and through these the sea water can freely reach 

 the interior of the vesicle. Two of these slits, which are opposite 

 each other, belong to the median plane. The other 4 lie interradi- 

 ally. On the sensory cushion within the vesicle rise 4 S-shaped 

 radially-placed springs ; these likewise consist of fused cilia, and their 

 free upper ends enter a globular mass of otoliths, Avhich they support. 

 From the 4 springs 4 rows of cilia run out through the 4 



