Prof. J. Miiller on the Structure of the Echinodemis. 11 



remained unknown. In the shell of an Echinus, the so-called 

 ambulacral plates are nothing but secondary ambulacral plates 

 made up of the minute primary" ambulacral plates. We may 

 observe on the inner side of the shell the sutures between the 

 primitive plates, which contribute to the formation of a secondary 

 ambulacral plate. The primary plates correspond in number 

 with the pairs of pores in one oblique transverse series of the 

 ambulacrum. The sutures are also recognizable upon the ex- 

 terior, and by careful examination we observe, that they divide 

 even the tubercles situated upon them, as may be well observed 

 in E. spheera. Where there are four pairs of pores in a second- 

 ary ambulacral plate, the four pieces have very unequal hori- 

 zontal diameters, the two median ones, in fact, being smaller 

 transversely ; all four pieces extend as far as the external suture, 

 but only the iii-st and the last attain the inner suture, applying 

 themselves around the ends of the shorter pieces in such a 

 manner, that the secondary ambulacral plate is divided inter- 

 nally by only one sutiu-e. The same suture exists when there 

 are three pairs of pores. Here, again, it is the plate of the 

 median pair of pores which does not reach the inner edge. In 

 E. albus, 9-10 primary plates for 9-10 pairs of pores form in a 

 similar way a secondary plate. The smaller plates are not sub- 

 sequent to and intercalated between, the larger, but are found 

 in the smallest specimens in which the plate is developed at all ; 

 so that a young Sea-urchin has just as many pairs of pores in 

 an obliquely ti-ansverse series as afterwards, according to the 

 species to which it belongs. 



Agassiz has demonstrated, that in Echinus the addition of new 

 ambulacral and inter-ambulacral plates takes place at the apical 

 end of the corona, but denies the occurrence of such new forma- 

 tions in the ChjpeasteridcB. The fact is, however, that it occurs 

 here in exactly the same manner as in the regular Sea-urchins. 

 The whole under surface of the ClypeasteridiB grows, retaining 

 the original number of its plates, only by their individual increase; 

 whilst the plates of the upper surface not only increase indivi- 

 dually, but at the same time fresh, minute, ambulacral and inter- 

 ambulacral plates are added at the apex. This must be borne 

 in mind in describing species, and the number of the plates 

 should be reckoned from the end of the petaloid ambulacrum to 

 the edge, and also from the edge to the mouth. 



Philippi, again, who confirmed and extended the observation 

 that new plates are formed at the apical end of the corona in 

 Echinus, denies that the same new development takes place in 

 the Spatangidee ; but it unquestionably occurs, from what I have 

 observed, in Schizaster canaliferus, at different ages. I have com- 

 pared specimens of 8"', of 2", and of 3". The first has in each 



