226 Dr. W. B. Carpenter on the Structure, [Jan. 20, 



Thus we are also to regard the arms and pinnules in the light of a 

 respiratory apparatus, an interchange of gases between the contents of 

 the canals and the surrounding medium being promoted by the subdi- 

 vision of the canals and the consequent extension of their surface. 



Nervo-muscular Apparatus. — In the First Part of my Memoir I de- 

 scribed the manner in which the contraction of the pairs of muscles 

 intervening between the vertical lamellae rising from the oral faces of 

 the successive calcareous segments of each arm coils it up into a spiral, 

 the uncoiling being produced, when the muscles relax, by the elasticity 

 of the ligaments interposed between the segments near their dorsal 

 margins. The coiling and uncoiling often take place through the whole 

 length of the arm, with a quickness and consentaneousness which has no 

 parallel in the comparatively sluggish and limited movements of other 

 Echinoderms ; and while a single arm may be made to coil up by irri- 

 tating one of its pinnules, the whole circlet of arms closes together when 

 an irritation is applied to the pinnules which arch over the mouth, an 

 act which affords a strong indication of the " internuncial" action of a 

 definite nervous system. The anatomical considerations alreadjr specified 

 having led me to the conclusion that the radial cords which pass through 

 the calcareous segments of the arms are really nerve-trunks, and that 

 the five-chambered organ in the centro-dorsal basin is their centre, I put 

 their character to the following experimental test, when visiting Oban in 

 the autumn of 1867 : — Having completely eviscerated a living specimen, 

 so that nothing was left but the calyx and arms, with the centro-dorsal 

 basin and its contents, I passed a needle down through the canal left at 

 the base of the calyx between the central faces of the first radials, so as 

 to irritate the quinquelocular organ beneath ; all the ten arms then sud- 

 denly and consentaneously closed up. On the withdrawal of the needle, 

 the arms gradually straightened themselves again, and again coiled up as 

 before when the irritation of the central organ was renewed. I am at a 

 loss to see in what respect the evidence thus afforded that this organ is 

 functionally a nerve-centre, and that the radial cords are nerve-trunks, 

 is less cogent than that which we draw from the contraction of the limbs 

 of a Vertebrate animal when we irritate the segment of the spinal cord 

 which gives off their nerves. The chief difficulty in assigning to them 

 such a function arises from their want of the histological characters of 

 nerves. The radial cords, hardened in strong spirit, may be torn into 

 fibrils of extreme minuteness ; but, so far as I have yet been able to 

 ascertain, these fibrils have a homogeneous protoplasmic composition. 

 This absence of differentiation may, as it seems to me, be related to the 

 fact that as the muscles are all flexors, the nerves have only one function 

 to perform, and that there is consequently no need of the insulation 

 which they require where nerve-fibres of very different functions are 

 bound up in the same sheath. 



The curious little saccular organs (ss, figs. 6, 8) which lie at intervals 



