NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK 3 



In the echinoderms the primary appendages, the teeth, the water 

 pores, and the genital openings, and in the crinoids the primary 

 nerve cords as well, are all interradial in position. There can be no 

 doubt, therefore, that the dividing lines between the five half somites 

 fall in the middle of the so-called radial areas, and in the echinoids 

 and asteroids a sharp line of division is always maintained down the 

 middle of the radial series of plates throughout life, while no such 

 dividing line is found in the interradial regions. 



In the young crinoids each of the somatic regions is completely 

 walled in by two large superposed plates, a dorsal basal and a " ven- 

 tral "' oral ; beneath the basals there are usually 3-5 small infrabasals 

 alternating Avith them and corresponding with the oculars of the 

 echinoids which, since they are entirely absent in large groups, and 

 are usually more closely associated with the column than with the 

 calyx, are probably to be interpreted as a dissociated columnal. 



The young crinoid therefore has its body protected by ten large 

 somatic shields, five dorsal and five ventral, the latter with the primi- 

 tive appendage under the median line. 



The arms first appear as evaginations in the intersomatic lines at 

 the plane of separation between the dorsal and ventral plates. The 

 evidence is that the skeleton of the arms is double, half being derived 

 from the somite on either side ; but whatever may be the ultimate 

 genesis of their skeleton, the arms arise as linear and almost im- 

 mediately biramous appendages taking their origin from the inter- 

 somatic planes. 



From this point onward the development of the animal is entirely 

 intersomatic ; the peristome and its underlying nerve ring, the water 

 tube ring about the mouth, the blood vascular ring, the genital cord, 

 and the coelomic cavities all send ofif radial branches which increase 

 in length as the arm grows, while along either side of the peristomial 

 extensions (ambulacral grooves) there is formed progressively a con- 

 tinuous series of reduplications (ambulacral lappets) of the little 

 flaps with their associated tentacles, the latter in communication with 

 the radial water tube, which surrounded the mouth in the " pre- 

 brachial '' stage. 



THE NERVOUS SYSTEM OF THE ECHINODERMS 



In the chordates the central nervous system never becomes sep- 

 arated by mesodermal tissues from the tract of ectoderm from which 

 it originated in the embryo. Sedgwick remarks that this is a feature 

 of all echinoderms in so far as the ventral nervous system is con- 



