178 



p. HERBERT CARPENTER. 



trace of them is visible in the adult, and the single anal 

 plate undergoes the same fate. The orals persist, however, 

 in Rhizocrinus and Htjocrinus. "VVyville Thomson^ describes 

 them in the latter genus (fig. xii) as five triangular valves 

 [or), which close over the central mouth so as to form a very 



sup 5 



'~?B, 



Tig. xii. — Diagram of the disc of 'Eyocrinus (adapted from a fio-ure of 

 Sir Wyville Thomson's). An. Anal tube. an. Anambulacral 

 plates. sup. Superambulacral or marginal plates = Saumplattchen. 

 or. Oral plates. R. Eadials. 



perfect five-sided pyramid. The peripheral part of the disc 

 is paved with closely-set plates {an), the outer ones of which 

 are in contact with the broad thin radials {H). The width 

 of this plated marginal zone surrounding the oral valves is 

 extremely slight, being only about one quarter of the total 

 diameter of the disc. The anal opening is near the edo-e 

 of this zone, at the end of a short, plated, interradial tube 

 (^An). The plates on the ventral surface of the disc to 

 ■which Miiller" gave the name of anamhulacral, are very 

 completely developed in many Pentacrini, as in Hyocrimis. 

 Many of them are perforated by the water-pores which place 

 the body-cavity in communication with the exterior. 



In Bhizocrinus, and in the Comatulce, these anambulacral 

 plates are less constant in their appearance than in Hyocrinus 

 and Pentacrinus. But in one or two tropical Coniatulce 

 they are very marked, and pass over at the margin of the 

 disc into a complete interbrachial plating, which covers the 

 surface of the perisome between the five rays, beyond the 

 level of the second radials. 



All this anambulacral system forming the peripheral part 



^ 'Journ.Linn. Soc.,' xiii, "Zoology," p. 53. 

 2 'Bau der Echinodermen/ p. 63. 



