ii CNIDARIA NERVOUS SYSTEM 93 



is so rich in sensory and locomotor organs. At this part in the 

 Craspedota a central apparatus, in the form of a double nerve ring, is 

 developed (Fig. 72, A and B, m\, ?w 2 , p. 95). One of these nerve rings, 

 the upper one, is placed on the exumbrellar, the other on the subum- 

 brellar margin, both close to the base of insertion of the velum. The 

 first innervates chiefly the sensory organs of the umbrella margin, the 

 latter the musculature of the velum ; the two are connected by fibrillw 

 running through the supporting membrane. A peripheral nervous 

 system is present in the form of a plexus of ganglion cells and fibres 

 connected with the lower nerve ring, especially on the subumbrella ; 

 it supplies the musculature of this part with nerves. The aboral 

 exumbrella, not only in the Craspedota, but also in the Acmspeda, is 

 devoid of a nervous system, as also of sensory organs and musculature ; 

 this is explicable by the ontogenetic and phylogenetic derivation of 

 these animals from a form attached by the aboral pole. 



In the Siphonophcrra also a plexus of ganglion cells at various parts 

 of the body has been demonstrated. 



In the Scyphomedusce or Acraspeda also a plexus of ganglion cells 

 is developed in the subumbrella. It is connected with considerable 

 central accumulations of nerve tissue on the subumbrellar margin. 

 These are developed in the Cubomedusce as 8 (4 perradial and 4 inter- 

 radial) ganglia, connected together by a circular nerve, from which 

 sensory nerves proceed to the rhopalia (sensory bodies) and to the 

 tentacles, and motor nerves to the musculature. In the Discomeduscc 

 there are also 8 ganglia at the bases of the 8 sensory bodies, but here 

 there is no connecting nerve ring. 



In the Corals a plexus of ganglion cells lying deep in the body 

 epithelium has also been demonstrated. This is particularly strongly 

 developed on the oral disc, and at the base of the tentacles. 



In the nervous system of the Ctenophora we can, according to recent 

 research, which still needs extension, distinguish the following parts : 

 (1) a diffuse, ectodermal, ganglionic plexus under the body epithelium, 

 which is spread over the Avhole surface of the body, and may be con- 

 tinued on to the wall of the oesophagus ; (2) fine, branching, nucleated 

 strands, which penetrate the jelly, and are connected with the muscles 

 of the jelly by lateral branches a connection between these strands 

 and the ectoderm is not yet proved ; (3) 8 meridional nerve strands, 

 running under the 8 ribs, and ciliated bands (see description of 

 sensory body), the rib-nerves. It is a remarkable fact that it has till 

 now been impossible to prove a connection between these three parts 

 of the conjectural nervous system and the sensory body at the ab- 

 oral pole. 



A fact which deserves special mention is that, in the Mcduscc 

 and Siphonophora, and especially in the Actinia, a more or less 

 extended plexus of ganglion cells may lie under the endodermal 

 epithelium. 



