418 Dr. H. Ludwig on the 



respect are beliind the three other radial vessels. Regarded 

 from outside, it is the anterior unpaired tentacle of the eighth 

 day of development and its neighbour on the left, which 

 belong to the left dorsal radial canal ; the tentacular vessel 

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

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

 are 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 

 possible to demonstrate it without meeting with a single 

 exception for all the numerous young Giicumarice of the most 

 widely difFeicnt ages, from the eighth to the hundred and 

 fifteenth day, in uninterrupted series of transverse and longi- 

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

 tl'.ough 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 opposite one 

 another with reference to the median plane of the Holo- 

 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 interradii. Previous to this 

 only a single tentacle existed in each interradial region sur- 

 rounding the mouth. Now, however, after the formation of 

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

 interradii 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 five 

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

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

 parous Ghiridota rotifera. Since in the adult ten-tentacled 

 Citcuviaria each radial vessel gives off two tentacular canals, 

 we may conjecture, as regards the further multiplication of 

 the tentacles, that the eighth arises on the left (dorsal) side 

 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 mentioning the fact that 

 tlie two ventral tentacles, although in the adult animal they 



