4 Echinoderinata. 



Preyers experimental investigations into the functions of the nervous sy- 

 stem in Asterids and Ophiurids have generally confirmed and also largely ex- 

 tended the results obtained by Romanes and Ewart [see Bericht for 1SS1 Ip 175]. 

 All points of the ambulacra! grooves and of the free dorsal integument are very 

 sensitive to stimulation, whether mechanical or electrical, chemical or thermal, 

 all forms of which give corresponding results , as shown by the movements of the 

 tube-feet. The effect of a weak and localised stimulus, whether ventral or dorsal, 

 is invariably a retraction of the feet ; but with a strong stimulus which produces 

 irradiation, the effects are different according as it is applied ventrally or dorsally. 

 In the latter case it produces a general extension of the tube-feet from the centre 

 outwards ; but irradiation of a ventral stimulus causes a centripetal and then a 

 centrifugal retraction , which is followed by an extension , either immediately 

 (Luidia) , or after repeated stimulation (Asterias) . Various facts show that the 

 peripheral reflex-agency is not so independent of the central organ in L. as it is in 

 the more sluggish and less sensitive species of Ast-rias and Astropecten, in which 

 the radial nerve cords are more autonomous. For the latter forms too the tem- 

 perature-maximum is higher than for L. The attachment of the feet depends on 

 a precise regulation of the pressure within the water-vascular system ; but the 

 regulating mechanism operates without the participation of the central nerve- 

 ring. Chemical substances, and especially nicotine, act unfavourably on the power 

 of attachment , and so does a rise of temperature above the maximum of the Gulf 

 of Naples (27), or stagnation of the water. The coordination of the creeping 

 movements of Asterias and Astropecten depends on the central ring, just as descri- 

 bed by Romanes in Uraster : and the dependence of the periphery upon the central 

 organ is still more marked in Ophiurids, all coordinated movements ceasing when 

 the origins of the five radial cords are pierced. Isolated rays crawl indifferently in 

 either direction, and ascend vertical surfaces. Righting movements are performed 

 more readily by Ophiurids than by Asterids, and take a longer time in larger in- 

 dividuals. In Ophiurids they depend upon the action of the central ring; but all 

 parts of an Asterid, peripheral or central, can right themselves when isolated. 

 The movements of an isolated ray become incomplete or casual when the radial 

 cord is divided in the middle. Those of larger pieces which are connected with a 

 central ring are more definitely purposive when two of its angles remain instead 

 of only one; i. e. two functionally equivalent parts of the nervous system are 

 more effective together than either is for itself. The righting movements are inhi- 

 bited by rise of temperature. Their cause is a purely central one, as they take place 

 without any peripheral reflex stimulation. They seem to depend on Retros.ub- 

 version-centres which are limited to the oral ring of Ophiurids, but also 

 occur in the radial cords of Asterids. - - Ophiurids adapt themselves very readily 

 to new and unnatural conditions ; and the expedients to which they resort in or- 

 der to liberate themselves from partial restraint seem to prove them to be posses- 

 sed of a rudimentary intelligence. This is far less evident in Asterids. 



Fraas describes the minute structure of the calcareous plates in Echino- 

 derms, more especially in the Asterids, and distinguishes between the irregular 

 network of those plates which serve merely for protection or support of other 

 organs, and the regular or oriented network. The latter is of two kinds 1) longi- 

 tudinal and radial, as in moveable pieces like the spines of Echinids; and 2) lon- 

 p'itudinal, as determined by the attachment of muscular fibres. This is especially 

 marked in the ambnlacral plates of Ophiurids, on the opposed surfaces of the 

 spines and. their tubercles in Echiuids, and in the brachials of Crinoids. It like- 

 wise occurs, together with radial differentiation, in the stems of Crinoids. 



