94 COMPARATIVE ANATOMY CHAP. 



VII. The Sensory Organs. 



We meet with special sensory organs for the first time in the 

 animal kingdom among the Cnidaria. The development and distribu- 

 tion of these organs is directly related to the manner of life. In 

 attached forms (Hydroids, Corals) there are only organs of touch (the 

 tentacles) ; in free-swimming forms (Acraspeda, Craspedota, Siplionophora, 

 Ctenophora) organs of smell, hearing, and sight may be added. 



The organs of touch are primarily the tentacles of which we have 

 already spoken. The sense of touch is due to special tactile cells 

 belonging to the body epithelium ; these are provided with a projecting 

 tactile hair, which is either flexible, mobile, or stiff*. The basal ends 

 of such cells are continued into nervous processes which are connected 

 with the nervous system. Tactile cells are to be found, not only in 

 the tentacles of the Cnidaria, but in great numbers on the margin of 

 the disc of the Medusce, and especially in the Ctenophora, scattered over 

 the whole free surface of the body. 



As organs of smell, or perhaps rather taste, we have : 



1. Small elub-shaped papillae, which in certain Leptomedusce are 

 found generally in great numbers at the edge of the umbrella between 

 the tentacles, being attached to the margin by thin stalks. They 

 contain a narrow blind canal, lined with thick cylindrical endodermal 

 epithelium, which comes from the circumferential canal. 



2. Pit-like depressions on the sensory bodies or rhopalia of the 

 Acraspeda, lined with a sensory epithelium which is much folded and 

 provided with long flagellate hairs. 



Auditory organs (perhaps also organs for regulating the position 

 of the body in the water) are found in Graspedote and Acraspede Medusce 

 and in the Ctenophora. 



We can distinguish three types of auditory organs : (1) auditory 

 vesicles, or marginal vesicles with ectodermal otoliths ; (2) tentaeulo- 

 eysts, or auditory tentacles ; and (3) the so-called sensory body of the 

 Ctenophora. 



I. The auditory or marginal vesicles are found in the division of 

 the Leptomedusce (Fesiculatce). These are, in the simplest cases, open 

 pit-like depressions of the subumbrellar epithelium near the base of 

 insertion of the velum. Within these auditory pits are one or more 

 otoliths, which have come from ectodermal cells, Avhile the cells which 

 form the base of the pit bear auditory hairs, on which the otoliths 

 rest (e.g. Mitrocoma). Auditory vesicles rise out of these auditory pits 

 by the closing of the pit, which moves towards the exumbrellar side of 

 the base of insertion of the velum and here forms an externally 

 rounded protuberance (Fig. 72, A). Auditory pits and auditory vesicles 

 receive their nerves from the subumbrellar nerve ring. In the simplest 

 cases we find 8 adradial auditory vesicles, but the number is often much 

 greater, and mounts up to many hundreds. 



