110 PROCEEDINGS OF THE AMERICAN ACADEMY 



five, when the pentagon has become closed. The opening of the anus, 

 on the contrary, has no such clear and precise position with reference 

 to the odd arm as the madreporic body. At any rate, it is perfectly 

 apparent that the madreporic body is always in the suture of the 

 terminal arms of the pentagon, which places it opposite the odd arm. 

 The case of the Echinometradae and of Salenidfe is constantly brought 

 up to show that the madreporic body is not connected with any definite 

 axis of the body ; but from what has been shown of the twisted state 

 of a young star-fish, and of the manner in which it unwinds itself after- 

 wards, and from the fact that in Echinidie we find families in which 

 the unwinding is not completed, and the madreporic body naturally 

 cannot be in a line passing through the middle of the animal, though 

 still opposite the odd arm whatever its position may be, owing to this 

 embryonic feature, as in the Echinometradas, we infer that the madre- 

 poric body retains the same normal position in all Echinoderms. 



On embryological grounds, from the changes which the young star- 

 fishes undergo, it is evident that all the star-fishes with pentagonal 

 outline and pointed tentacles, like Ctenodiscus, stand lower than star- 

 fishes like Luidia, with elongated arms and pointed tentacles ; that pen- 

 tagonal star-fishes, like Culcita, without any sjiines or plates, though 

 higher than those just mentioned on account of the discs of the tentacles, 

 stand lower than those pentagonal star-fishes like Hippasteria and An- 

 tennea, where we have a complicated system of plates. Next, those 

 star-fishes which have suckers and long, smooth arms, like Ophidiaster, 

 stand lower than those star-fishes which have tentacles provided with 

 suckers, and complicated spines on the surface of their long, slender arms, 

 as Asteracanthion. Finally, as a general rule, all star-fishes with two 

 rows of suckers are lower than those which have four, the former 

 being an embryonic character. Among other characters which are 

 not those of the order, but nevertheless exist in the young star-fish, 

 the most prominent is the position of the madreporic body on the 

 actinal side, which is a feature of the Ophiurans. The position of 

 the anus next to the mouth is eminently crinoidal, as well as the 

 arrangement of the plates on the abactinal side ; while the arched 

 abactinal area and the tall spines remind us of the Echinoids. 



The mode of development of Echinaster is not according to a dif- 

 ferent plan ; it seems to me to be only a shorter way of arriving at the 

 same result. There is not the same complicated system of arms, as 

 the young is not nomadic, but is carried about by the parent. From 



