528 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



subepithelial enteric plexus, whicli has been made out by the author in 

 Crinoids and Asteroids. 



Morphology of Crinoids.* — Dr. 0. Hamann, in a preliminary com- 

 munication, deals with the nervous system of Crinoids. The epithelial 

 portion consists of the subepithelial plexus described by Ludwig and 

 others, and of the central oesophageal ring ; it corresponds to the oeso- 

 phageal ring and ambulacral nerves of other recent Echinoderms, which 

 are partly epithelial and partly mesodermal in position. In Crinoids the 

 epithelium of the ambulacral grooves is considerably thickened, and 

 consists of the same elements as in a starfish. In both there are epithelial 

 nerve-fibres with bi- and multipolar ganglionic cells ; they have not 

 lost their connection with the epithelium of the body; this nervous 

 system has no central ring. The epithelium of the grooves is made up 

 of sensory and supporting cells, and the processes of the latter traverse 

 the nerve-fibrous ring vertically. In each tentacle there is a nervous 

 band which innervates the sensory papillae. 



Another system of nerves is placed in the connective substance, 

 and to this belongs the fibrous mass placed around the chambered organ 

 with its nerve-trunks, which run along the dorsal side of the arm. 

 Another part of it is ventral in position, and has its own central organ. 

 The two portions of this mesodermal nervous system are connected with 

 one another. The ventral or oral part is divisible into a central organ, 

 an oesophageal ring, and the nerves given off from it ; some of these 

 nerve-trunks have been described by Carpenter as a periambulacral 

 network. The nerve-fibrils of the ring have a concentric course, and 

 the number of nerves given off is very large. Some of these take a 

 dorsal or aboral direction and branch in the mesenteries and bands of 

 the ccelom, and on the organs that lie therein. Other nerves pass into 

 the circumoral tentacles. Those that are of the greatest interest are 

 those which enter into definite relations with the' water- vessels ; these 

 are, at first, five, and they bifurcate and pass into the oral body-wall of 

 the arms. Each water-vessel is accompanied on either side by a 

 nerve-trunk, so that they are twenty nerve-bands in the ten arms, to the 

 tips of which they may be traced. They likewise pass with the 

 branches of the water-vessels into the pinnulte. 



The dorsal or aboral portion of the mesodermal nervous system has 

 likewise a central organ, and the course of the fibres which compose it 

 is complicated. It gives off solid nerve-trunks into the arms, and never 

 hollow tubes as some observers have asserted. The trunks in the arms 

 give off branches from four opposite points ; some of these go to the flexor 

 muscles, and others, after much branching, to the dorsal epithelium. 

 Between every two groups of muscles, nerves pass out which go almost 

 directly to the oral body-wall, where they become connected with the 

 oral pair of longitudinal nerves which belong to the oral part of this 

 mesodermal nervous system. 



Both the epithelially and mesodermally placed systems are com- 

 posed of very fine fibres, which generally run parallel and in cross 

 section appear dotted, and of ganglionic cells of various types. The 

 nerves of the cirri are regarded as special nerves ; they are the only 

 vascular nerves found in Crinoids. 



In addition to the sensory papillae of the skin there are nerve-end- 



* Nacbr, K. Gesell. Wiss. Gottingen, 1888. pp. 127-31. 



