Dr. Carpenter on Antedon rosaeeus. 201 



referred. And he directs special attention to the curiously inflected 

 rosette-like plate, previously unnoticed, which occupies the central 

 space left within the annulus formed by the adhesion of the first 

 radials. This plate is in special relation to the organ termed by 

 Job. Miiller the " heart," but certainly having no proper claim to 

 that designation, being a quinquepartite cavity in the central axis, 

 from the walls of which there pass out not vessels but sohd cords 

 of sarcode, into the rays and arms, and also into the dorsal cirri. 

 The inflexions of the rosette-like plate serve for the support and 

 protection of the large cords passing into the rays, each of which has 

 a double origin, and a connexion with the adjacent radiating cords 

 that reminds the anatomist of the " circle of WilHs." 



The skeleton of the adult diff^ers so widely in the forms and rela- 

 tions of its parts from that of the early Pentacrinoid larva described 

 by Prof. Wyville Thomson, that the derivation of the former from 

 the latter can only be understood by observation of all the inter- 

 mediate stages. When the calcareous skeleton of the calyx first 

 shows itself, it consists only of five oral plates arranged conform- 

 ably upon five basal plates, as thus : — 





 B B B B B 



At a stage a little more advanced (which has been described by 

 Prof. AUman, Trans. Roy. Soc. Ed. vol, xxiii. p. 241), the rudiments 

 of the/r«^ radials are found interposed between the orals and basals, 

 alternating in position with both, as in the 

 margin ; and between two of these first radials 

 there appears a single small unsymmetrical a a a a a 

 plate, which afterwards proves to be the anal. B B B B B 

 The first radials undergo a rapid increase in 

 size, and soon become surmounted by second and third radials, 

 which project between the orals ; whilst the orals and basals, under- 

 going no such increase, are relatively very much smaller ; the anal 

 plate is still found on the line of the first 



radials. But from this time the radials A^ A^ A' A' A^ 

 form the principal part of the calyx, A^ A' A* A* A' 

 which opens out widely in conformity 

 with the increase of space required for A^ A'anA^ A^ A^ 

 the digestive apparatus, the intestinal B B B B B 

 canal being now developed around what 



was originally a simple stomach with one orifice. The highest joint 

 of the stem also undergoes a remarkable increase in size, and begins 

 to acquire the form of a basin with an inflected rim, constituting 

 what is known in the adult as the centro-dorsal piece. "When the 

 calyx opens out, the five oral plates, which originally formed a circlet 

 around the mouth, retain that position, and detach themselves en- 

 tirely from the divergent radials, nothing but the soft perisomatic 

 membrane filling up the space between them. These oral plates 

 never increase in size, and towards the end of the Pentacrinoid stage 



