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 the oral cavity, be- 

 tween the oesophagus and the tips of the 

 ossicles of the masticatory apparatus, 

 and is attached by five pairs of bands 

 in this position. The nerve-trunks (c) 

 pass from the angles of the pentagon 

 to the spaces between the pieces of the 

 pyramid; and extend thence over the 

 oral integument to the ambulacral areee. 

 They are greatly widened in the middle 

 of their course, and divided into two 

 lateral halves by a median groove. The lateral branches from the 

 principal trunks accompany the branches of the ambulacral vessels. 

 The nervous system has the same arrangement in the Spatangidte, 

 but in them the oral ring forms a pentagon with unequal angles. 



The nervous ring of the Holothuroida lies just in front of and a 



Pig. 106. Nervous system of 

 Echinus lividus; tlie masti- 

 catory apparatus is removed. 

 a CEsophagus cut througli trans, 

 versely. b The commissures 

 of the nerve-trunks, forming 

 a pentagonal oesophageal ring, 

 c The nerve-trunks passing to 

 the radii, d Bands which hold 

 together the tips of the pyramids 

 of the masticatory apparatus 

 (after Krohn). 



