NERVOUS SYSTEM OF COELENTEEATA. 109 



ectoderm, in the' Hydroid-Polyps, is as yet an ^differentiated 

 organ of sensation. Irritations of it produce movements of the 

 fibres of the muscular layer connected with the cells (<§ 25) ; and 

 it is only in the Medusas that distinct parts can be recognised as 

 belonging to a nervous system. These form a ring, which runs 

 round the edge of the disc, and which is formed of a fibrous 

 tissue, on which ganglionic swellings, formed of cellular, elements, 

 are placed at regular intervals. The ganglia correspond in position 

 to the marginal bodies, which are to be considered as sensory 

 organs, and send off fibres which pass partly to the tentacles, and 

 partly accompany the radial canals. This nerve-ring, which is 

 most accurately known in the Geryonidee, is supported on the 

 annular cartilage, and lies between it and the circular canal at the 

 edge of the disc. The swellings of the nerve-ring represent central 

 organs, which are connected with one another by the fibrous portions. 

 From experiments also in which the edge of the disc was divided, 

 it seems clear that there is a central nervous system in it. 



The nervous system of the Ctenophora is as yet not well known. 

 As to the rest of the Acalepha?, no organs of this kind are known 

 with any certainty. 



Sensory Organs. 

 § 85. 



Owing to the imperfection of our knowledge of the nervous 

 system of the Ccelenterata, no definite opinion can be given as to the 

 parts which are to be regarded as sensory organs. This remark 

 refers as much to the arrangements which we regard as subserving 

 the sense of touch as to higher sensory organs. Special pi'ocesses 

 of the body appear to serve for the general tactile sense which is 

 present in the integument, and these we have already spoken of as 

 tentacles (§ 79). Whether there are, on the other hand, special 

 organs, must for the present remain undecided ; although the 

 presence of stiff seta3 on the tentacles, and around the mouth, 

 leads us to admit the existence of distinct organs of touch. 



More differentiated organs, adapted for sensory perceptions, 

 are found in the so-called " Marginal bodies/' which are attached 

 to the edge of the umbrella in the free Medusae, and which are of 

 two distinct kinds. The first have the appearance of vesicular 

 structures, the second are collections of pigment provided with a 

 transparent refracting body, similar to those organs which, in the 

 higher animals, are seen to be the terminal organs of the optic 

 nerves. The former, or marginal vesicles, are either embedded 

 in the substance of the disc, or project freely at its edge. They 

 consist of a homogeneous capsule, covered with epithelium, and 

 enclose one or more concentrically striated concretions, or small 

 crystals. The concretions are in close relation with the wall of 

 the vesicle, being encased in a spherical outgrowth of it. As 



