'28 



EMJIRYOLOGY. 



cannot fail to come to the conclusion that it is the mouth which is turned 

 towards us in both cases. If Midler's statement were correct, we should have 



Fig. 63 



Fir. fi4 



the anomaly in young 

 sea-urchins of finding 

 all the tentacles placed 

 between the spines 

 and the anus, on the 

 abactinal side of the 

 test, while on the ac- 

 tinal area we should 

 have nothing but a 

 closed membrane. 

 This is so contrary to 

 the plan of develop- 

 ment of Echinoderms, 

 whether Echinoids, 

 starfishes, or Ophiu- 

 rans, from the obser- 

 vations of Midler him- 

 self, that I give the 

 above explanation, 

 which seems to bring the figures of Midler in 

 accordance with what I have observed. The ap- 

 pearance of the teeth, in Midler's figures, on what 

 seems tin' abactinal side, i- due to compression 

 also. The spines of the young sea-urchins ob- 

 served by Midler have a very uniform appear- 

 ance : they are nearly all hexagonal prisms in 

 their earliest stages. The same is the case with 

 our young sea-urchins, though they lose their 

 embryonic character at an earlier period than is 

 the case in any species observed by Midler. 



We know unfortunately too few of the embryos 



of Echini to be able to compare the pluteus of 



the different orders. From the little we know 



- we find very extensive differences. The pluteus 



of the Spatangoids, as far as we know them from 



