372 Dr. Agassiz's Observations on Echinodermata. 



terambulacral series to another, so that those which lie in a 

 vertical line one upon another, do not succeed each other in 

 the order of their first growth. 



The growth of the plates of Echini takes place chiefly at 

 the upper summit of the shell, and the plates enlarge from the 

 top towards the bottom until they arrive at the greatest cir- 

 cumference of the spheroid, where they become entirely con- 

 solidated : at the lower summit, i.e. around the mouth, the 

 growth only takes place during the youth of the animal. 

 Hence arise the differences of form which are observable be- 

 tween the young and the old Echini. In youth they are com- 

 paratively flat, and become more and more conical as they 

 advance in age. In the Starfish the new plates are found in 

 the angles of the radii nearest to the upper surface and lower 

 surface of the body ; and increasing more and more, they keep 

 carrying to a greater distance the extremity of the radii. Thus, 

 then, the number of the plates is continually increasing, and 

 cannot be employed as a specific character. Hence we see 

 how an Echinus or a Starfish receives its increase, still pre- 

 serving its essential form and the relative disposition of its 

 parts. 



The same system prevails in the structure of the Crinoidea. 

 In their internal organization it is well known that these animals 

 had between their five rays a depression, which no doubt con- 

 tained their soft organs; but the nature of this cavity is in 

 part unknown, because the interposition of the rays usually 

 obscures the view of their central region ; yet after a close 

 examination of many specimens of Br i avian Pentacrinite, I 

 have found that the branching rays are disposed around an 

 inclosed cavity, having walls composed of plates distinct from 

 those of the rays. The summit of this pouch presents an 

 aperture surrounded with certain plates stronger than the 

 rest. This aperture is the mouth, with its manducal plates. 

 in many of the Crinoidea, particularly in the Actinocrinites, 

 I have observed on the side of this cavity a second large 

 aperture, placed between two rays of the animal. I consider 

 this to be the anus ; if so, this aperture occupies the same 

 place as in the P. europceus, and presents a new resemblance 

 to it. These data are sufficient to determine the bilateral dis- 

 position of the parts in the Crinoidea, as in the Echini and 

 Astoria, and to point out which of the rays are pairs, and 

 which is the odd ray. The differences which exist between 

 them are so great, that in certain genera, e. g. in the Pentre- 

 mites and Actinocrinites, they are obvious at the first glance. 



As for the genera established in the whole class, I have 



