ZOOLOGY AND BOTANY, MIOKOSCOPY, ETC. 407 



take the study of Echinoderms by the reflections raised by the observation 

 of the physiology of the embryos of higher animals, many movements of 

 which appear to be long inherited. After enumerating the twenty-one 

 species on which he experimented, he speaks shortly of the work of previous 

 observers, and expresses, a propos of the papers of Messrs. Romanes and 

 Ewart, the very reasonable desire that physiological works should state 

 definitely the species on which experiments were performed. 



The'^ ambulacral pedicles of Asterids are polydynamous organs of a 

 special character; their mobility and sensitiveness, their large number, 

 suctorial function, together with their locomotor and respiratory significance 

 cause them to be of great interest. It was especially interesting to in- 

 vestigate the causes of the retraction and erection of these organs, which 

 are capable of all kinds of vermiform movements and twistings ; so long 

 as the animal is quite fresh and normal, extension is more rapidly effected 

 than retraction. The fundamental phenomenon is that which was long 

 'since mentioned by Tiedemann — the retraction of the suckers on slight 

 mechanical irritation ; dorsal irritation of a ray is almost but not quite as 

 speedily followed by retraction of the suckers underlying the region 

 touched ; the extent of irradiation, or distance to which the stimulus exerted 

 its effect, was found to vary with different species, but it was generally 

 found that, in all five-rayed star-fishes, there was an almost simultaneous 

 retraction of the central feet of the two neighbouring rays, and in very 

 many cases of the remaining two later on. Chemical stimuli have a 

 generally similar effect to mechanical stimuli. 



Isolated rays were next examined and confirmed Romanes' observations ; 

 but differences were observed with different species, Luidia not responding 

 as well as Asterias, and indicating that in it peripheral reflex actions were 

 not so much independent of the central organ as in the less mobile and 

 otherwise less sensitive species of Asterias and Astropecten ; in the latter 

 the radial medulla (an ambulacral spinal cord in the physiological sense) 

 is more autonomous or less dependent on the ambulacral brain, or central 

 nerve-ring with its rich supply of ganglionic cells. 



With regard to the extension or erection of the suckers, it was found 

 that strong centrodorsal mechanical irritation extended centrifugally into 

 all the rays, but when it is weak the effects may be confined to the 

 circumoral feet, and be transitory. The extension of the suckers after local 

 dorsal stimulation is always a central process ; if the centres ai^e wantinof or 

 injured, and their connections broken, the extension is affected. The details 

 of experiments with electrical and thermal stimuli are also given. 



It is clear from these experiments that — 



(1) If, on an uninjured star-fish, a local ventral or dorsal stimulation 

 exerts only a local effect there is always a retraction, and never an extension 

 of the ambulacral feet. 



(2) If a local dorsal stimulus irradiates, a general extension from the 

 centrum follows, and never a retraction ; thus : — 



Place of stimulation. Result. 



Dorsal. ) Local : Retraction. 



Excentric or " dorsal " ( Irradial : Extension. 



Ventral. ( Local : Retraction. 



Excentric or central ) j ^ • i . ^ Retraction. 



Extension. 



■■{ 



After describing the phenomena of attachment to foreign bodies, of 

 creeping, and of climbing, the author enters upon an interesting account 



