Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 NEEVOUS SYSTEM OF ECHINODEEMA. 209 form-elements enter into the formation of these nervous tracts^ and have the function of a supporting tissue, points to this conclusion. In the Asterida each radial nerve consists of two bands thickened in the middle, in which cellular and fibrous elements are equally distributed. At the ends of the arms the radial nerves form a large swelling, which is connected with the optic organs placed there. In Comatula this nervous band has the same characters. It
Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 NEEVOUS SYSTEM OF ECHINODEEMA. 209 form-elements enter into the formation of these nervous tracts^ and have the function of a supporting tissue, points to this conclusion. In the Asterida each radial nerve consists of two bands thickened in the middle, in which cellular and fibrous elements are equally distributed. At the ends of the arms the radial nerves form a large swelling, which is connected with the optic organs placed there. In Comatula this nervous band has the same characters. It is accompanied by a blood-vessel, which is placed in the middle of it, and, being pushed into it from above, divides it into two halves. Branches are given off in regular order to the pinnules. In the Ophiurida the radial nerve-trunks (Fig. 100, B n) are placed in a space, covered over by the ventral plates (6), and supported by a layer which, by its continuation into the ambulacral feet, is shown to belong to the integument. In many, however (Ophiura testurata), the nerves themselves are considerably differentiated. They each consist of two nerve-chords, in which masses of ganglionic cells are deposited; these correspond to the metameres of the arms. The longitudinal trunks are connected by transverse commissures with these ganglia, from which peripheral nerves are given off. Each radial nerve therefore represents a ventral ganglionic chain. The connection between the nervous system and the integument, although at first sight this is implied only by the position of one on the other, is an important aid to the comprehension of the relations of the skeleton. For when this arrangement obtains the ambulacral groove cannot be calcified; this can only happen when the nervous system becomes more inde- pendent. In the Echinoida provided with a masticatory apparatus, the nerve-pen- tagon is intimately connected with it. In Echinus (Fig. 106) the nerve-pentagon lies above the fioor of
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