518 COMPARATIVE ANATOMY chap. 



The radial canals. — The five radial canals do not develop with equal rapidity, 

 nor indeed do the radial nerves and the radial longitudinal muscles. The medic- 

 ventral organs (radial canal, radial nerve, longitudinal muscle) in all cases arise first, 

 then follow the organs of the two dorsal radii, and only at a later stage, those of 

 the two lateral ventral radii. 



The tube-feet. — In agreement with the order of appearance just described, the 

 two first tube-feet, already mentioned above, belong to the ventral radius (Fig. 

 415). The two next in order also belong to the medioventral radial canal, and, 

 according to the rule which applies to all the newly developing tube-feet, arise in 

 front of those already present. The fifth tube-foot belongs to the left dorsal radial 

 canal. (The correspondence of this order with that of the rudiments of the tentacles 

 should be noted. ) 



According to observations which have been made, it appears that 

 when, in a Holothurian, the tube-feet are scattered, this arrangement 

 is, ontogenetically, secondary. In the same way animals which have 

 several rows of tube-feet in each radius have only two rows in a young 

 stage, or a zigzag row of alternating feet. 



The nervous system. — The first part of the nervous system to 

 appear is the oral circular nerve, and this arises as an ectodermal 

 circular ridge on the floor of the oral vestibule in the larva. It 

 sends out five band-like processes in the direction of the rudiments of 

 the radial canals ; these are the rudiments of the radial nerves. 



In that the rudiments of the circular nerve and the radial nerves 

 become subepithelial, there arises between them and the body 

 epithelium which closes over them a narrow space ; this is the 

 epineural canal. 



The rudiments of the five radial nerves grow backwards together 

 with those of the radial canals. 



In Cucumaria Planci there seems to be no larval nervous system. 

 In iSi/najita, on the contrary, the larval nervous system yields the 

 rudiments of the definitive system. The two lateral nerve ridges of the 

 Aurinilaria larva, when the oral vestibule of the barrel-shaped larva is 

 formed, shift into it. Their free ends then become connected from 

 the two sides to form a ring encircling the mouth, which is the rudi- 

 ment of the nerve ring. 



The Intestine shows, at an early stage, the coils characteristic of the adult. 



The first portions of the calcareous ring to appear are the five radial pieces : these 

 arise on the radial canal and, like all the calcareous structures, are produced by the 

 mesenchyme. The medioventral calcareous piece is from the first the largest. 



The enterocoel. — The right and the left enterocn'l vesicles grow round the intes- 

 tine. At the point where they meet ventrally they open into one another. 

 Dorsally they press the mesenchyme cells together in a vertical lamina. In this 

 way the dorsal (anterior) mesentery arises. The middle and the posterior mesen- 

 teries probably arise in consequence of the two enterocoel vesicles twisting round the 

 intestine posteriorly. 



The visceral wall of the enterocal, which becomes applied to the intestine, 

 presses the mesenchyme cells, which have greatly increased in number, against the 

 endodermal intestinal wall, till they form a layer, which becomes the connective 



