418 Dr. II. Liidwiff on the 



G 



respect are beliiiid the three other radial vessels. Rt^garded 

 from outside, it is the anterior unpaired tentacle of the eip:hth 

 day of development and its neighbour on the left, which 

 belong to the left dorsal radial canal ; the tentacular vessel 

 on the riglit of the unpaired one belongs to the right dorsal 

 radial canal ; tlie two tentacles of the posterior pair, however, 

 arc those which are furnished from the median ventral radial 

 canal. 



The relation of the primary tentacles to the radial vessels, 

 which has just been described, is perfectly constant. It was 

 ])0ssible to demonstrate it witliout meeting with a single 

 excejition for all the numerous young Cucximarue of the most 

 widely diffeient ages, from the eighth to the hundred and 

 iiilecnth day, in unintcrru])ted series of transverse and longi- 

 ludinal sections, and may therefore be regarded as a rule, 

 though certainly a very peculiar one. 



It was not until the hundred and sixteenth day that among 

 a portion of the young animals an increase of tentacles took 

 place, and seven altogether were found to be present. The 

 sixth and seventh tentacles are situated exactly op])Osite one 

 another with reference to the median ]dane of the IIolo- 

 thurian, and receive their water-canals from those two radial 

 vessels, which hitherto had taken no part whatever in the 

 giving off of tentacular vessels, namely from the right and 

 left ventral radial vessels. The two radial vessels each send 

 off the new tentacular vessel in a dorsal direction, therefore 

 into the left and right dorsal intcrradii. Previous to this 

 oidy a single tentacle existed in each interradial region sur- 

 rounding the mouth. I^ow, however, after the formation of 

 the sixth and seventh tentacles, each of the two latero-dorsal 

 iritcrradii possesses two, while the median dorsal and the two 

 ventral interradii now as before each accommodate only one. 

 The seven tentacles are accordingly disposed upon the Hve 

 interradii in precisely the same way as that which I deter- 

 mined years ago in the seven-tentacled young of the vivi- 

 parous Chiridota rotifera. Since in the adult ten-tentacled 

 Cvcumaria each radial vessel gives off two tentacular canals, 

 we may conjecture, as regards the i'urther multiplication of 

 the tentacles, that the eighth arises on the let't (dorsal) siile 

 of the right dorsal radial vessel, the ninth and tenth, however, 

 on the ventral side of the left and right ventral radial vessels, 

 whereby an exactly radial distribution of the ten tentacles of 

 the adult animal is finally attained. In connexion with the 

 successive development of the tentacles which has thus been 

 traced, it may also be worth while nuntioniiig the fact that 

 the two ventral tentacles, although in the adult animal they 



