ON THE EMBRYOLOGY OF ECIII^ODERMS. 13 



among the many tentacles the original odd tentacle which was so prominent in the 

 younger stages. Neither have I succeeded in determining the position of the eye in 

 any of the stages of these young Sea-urchins, owing to the early presence of the spines 

 and of the large pigment-cells, which prevent us from obtaining a favorable view of 

 the odd terminal tentacle in the young forms ; neither liave I been able to satisfy 

 myself Avhether this odd terminal tentacle retains its original position during the whole 

 life of the Sea-urchin, as is the case in the Starfish. I can likewise say nothing con- 

 cerning the development of additional ambulacral tentacles. 



The figures of young Sea-urchins which ^Miiller has given belong unfortunately nearly 

 all to different suborders from our Toxopneustes, so that we cannot make the com- 

 parison with our young Sea-urchins as close as we might wish ; and besides this, the 

 figures of Miiller are not drawn in such a way as to discriminate between the parts 

 which belong to the ambulacral and interambulacral spaces. This is particularly evi- 

 dent in his figures from the mouth side, Avhere we frequently find tentacles in such 

 numbers as must make the development of the difierent ambulacra unequal. The 

 same is the case with the spines. Any one who will take the trouble to compare the 

 figures of young Sea-urchins of Plates IV. and VII. of his first Memoir, Plate VII, 

 fourth Memoir, and Plate IV. seventh Memoir, with tlie figures given here (figs. 24 - 

 28), will see that, although they agree in their general characters, yet it is impossible to 

 place the different spines or tentacles in such positions that they will be divided into 

 ambulacral and interambulacral regions, which is easily done with the figures I have 

 given. We must remember, however, that most of Miiller's figures are Clypeastroids 

 and Spatangoids, which may make it difficult, if not impossible, to divide the young 

 Sea-urchin into ambulacral and interambulacral areas, where we have nothing like 

 regular vertical rows to guide us, as in our Toxopneustes. One great difference, how- 

 ever, will strike us at once ; it is, that what Muller has called anus I have in my 

 figures called mouth. The view he has taken is probably due to the fact that the 

 young Sea-urchins from which he made his drawings were compressed. Having fol- 

 lowed the mouth in the different stages which have been represented here, I think 

 there can be but little doubt that ]\Iiiller was mistaken. Compare fig. 26 of this 

 paper with his fig. 3, Plate VII. first Memoir, and we cannot fail to come to the con- 

 clusion that it is the mouth which is turned towards us in both cases. If Miiller's 

 statement were correct, we should have the anomaly in young Sea-urchins of finding all 

 the tentacles between the spines and the anus on the abactinal side of the test, while on 

 the actinal area we should have nothing but a closed membrane. This is so contrary 

 to the plan of development of Echinoderms, whether Echinoids, Starfishes, or Ophiurans, 



