REPORT ON THE CRINOIDEA. 115 



PL LX. fig. 2—a.d.; fig. G, a'. Figs. 4, 5, 7, «'; fig. 8, a.d.), where muscle fibres are absent, 

 indicates that if it be nervous, it must be not only of a motor but also of a sensory nature. 

 This conclusion also follows from the fact that stimulation of one of the oral pinnules of 

 Antedon rosacea causes the flexion of all the ten arms. This result is not simply due to 

 general irritability ; for if so, it would follow when any pinnule was stimulated ; whereas 

 stimulation of one of the ordinary pinnules is only followed by flexion of the arm which 

 bears it. This experiment therefore is evidence of a reflex action of a somewhat complex 

 nature ; and the axial cords must be the paths of both afferent and efi"erent impulses. 

 For there is no ambulacral nerve in these oral pinnules, which resemble those on the 

 hinder arms of Actinometra (PL LVL fig. 7 ; PL LXI. fig. 3) in being ungrooved, and 

 devoid of tentacles, blood-vessels, and ventral nerve (fig. 4). The latter is normally 

 connected with the tentacles, and possibly also with a general subepidermic plexus ; 

 but it has nothing whatever to do with the bodily movements of the animal, though 

 perhaps influencing those of the tentacles and of the marginal leaflets or covering plates. 



Here then we have evidence in the Crinoid of a mesodermic nervous system analogous 

 to that which has been discovered of late years in the Ccelenterates, Worms, and 

 Chsetognatha. As regards the latter group, 0. Hertwig is inclined to think that " bei 

 den Chsetognathen sensibles und motorisches Nervensystem von einander vollstiindig 

 gesondert sein, ersteres wiire ektodermal, letzteres gleich den Muskeln mesodermal." ^ 



Considering that nervous tissues are well develo^jed in the mesoderm of Ccelenterates, 

 one would certainly exj^ect to find them in that of Echinoderms. The nervous system of 

 a Holothurian only remains in connection with the epidermis at the distal ends of the 

 entacles and tube-feet ; w^hile the radial nerves of Ophiurids and Urchins are separated 

 from the exterior by limestone plates, though coming into connection with the epidermis 

 on the tube-feet. Besides the subepidermic plexus on the outside of the shell of an 

 Urchin which sends fibrils to the muscles of the pedicellarise, there is another which is 

 formed by filaments that are given oS from the lateral branches of the radial nerves, the 

 connection of which with the subepidermic plexus has not been definitely traced. 

 Eomanes and Ewart have further discovered that the general co-ordination of the spines 

 for the purpose of locomotion depends on the integrity of an internal nervous plexus 

 which is " everywhere in intimate connection with the external, apparently through the 

 calcareous substance of the shell." ^ 



There is, therefore, no very great difiiculty involved in the belief that a mesodermic 

 nervous system is present in the Crinoids. The morphological difliculties resulting from its 

 anti- ambulacral position are, however, considerable. But they are of precisely the same 

 character as we have to face, when describing the chambered organ and the vascular axis 

 of the stem as a part of the circulatory system of a Crinoid. Ludwig appears to hiwc 



^ Die Chcetognathen, Jenaische Zeitschr., BJ. xiv. p. 234. 

 2 Phil. Trans., 1881, p. S74. 



