Prof. J. Miiller on the Structure of the Echinoderms. 19 



lie in the sutures : in Clypeaster and Arachnoides, where the am- 

 bulacral plates are alternately dissimilar, so that the internal 

 area of the ambulacrum is formed by the broad plates, the loco- 

 motive pores are found in the plates themselves, so arranged, that 

 either two transverse series of pores occur on one plate {Clypeaster 

 Rangianus and placunarius and Arachnoides placenta)^ or, as in 

 Clypeaster rosaceus, four series of pores exist upon one ambu- 

 lacral plate. In the former, therefore, every great ampulla of the 

 branchial feet receives the lateral vessel of one series of pores and 

 small ampullae ; in the latter, two series of small pores correspond 

 with every large pore. Clypeaster rosaceus has (judging by a 

 hasty examination) about 4000 of these fine locomotive pores 

 within each petaloid ambulacrum, CI. Rangianus about 1200, 

 CL placunarius about 600. In Echinarachnius parma their external 

 apertures also lie in the sutures, and likewise in regular transverse 

 series, which is rendered possible by the extreme minuteness 

 of the tubercles upon the shell of this sea-urchin. In the Cly- 

 peasters with large tubercles, on the other hand, the latter dis- 

 turb the serial arrangement of the pores upon the external surface 

 of the shell, and the apertures may rather be said to be scattered 

 irregularly between the tubercles. 



In Clypeaster placunarius we meet with small calcareous spi- 

 cula on the inner surface of each ambulacral plate between the 

 series of pores : in CL Rangianus these spicula, which also abound 

 over the rest of the internal surface of the shell, are arranged 

 upon walls, into which the ambulacral plates are produced be- 

 tween every two series of pores. In Clypeaster rosaceus the spi- 

 cula are absent ; on the other hand, the walls of the ambulacral 

 plates are raised up into septa, which are again united by a cal- 

 careous covering, in which the sutures of the ambulacral plates 

 are repeated. The petaloid ambulacra, therefore, have double 

 walls (like the dome of St. Peter's at Rome), the inner wall be- 

 longing only to the locomotive area, not to the area of the large 

 or branchial pores, whose ampullae are uncovered. Between the 

 double walls in these Sea-urchins there are regular interseptal am- 

 bulacral chambers, each of which contains four rows of locomotive 

 pores. All the transverse chambers are, however, intersected by 

 a longitudinal passage, open towards them, for the median ambu- 

 lacral vessel. In the covering there are three longitudinal series 

 of apertures, by which the ambulacral galleries are connected 

 with the abdominal cavity. The middle series answers to the 

 median gallery, the lateral ones to the transverse chambers ; the 

 lateral apertures are intended for the passage of the ambulacral 

 vessels to the large ampullae of the ambulacral gills. Since every 

 chamber opens in the neighbourhood of two double pores, the 



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