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



investigation impossible, and allows only of dissection under the 

 simple lens, by whicli tbe constitution of the plates and the am- 

 bulacral pores may be very well detected, but the relation of the 

 feet to the ambulacral vessels is not directly observable. The 

 entire inner surface of the calyx is loosely lined by a membrane, 

 which also contains very minute microscopic calcareous plates. 

 In making a general comparison of the ambulacra in the differ- 

 ent orders, it is important to remember that Pentacrinus pos- 

 sesses not only median azygos but also lateral conjugate ambu- 

 lacral plates, and that the ambulacral pores lie between the two. 

 The ambulacra of Pentremites so far agree with this, that, ac- 

 cording to Roemer's exact analysis, they possess besides the con- 

 jugate plates, a median azygos plate, which however runs under 

 the whole ambulacrum. The ambulacral vessel and its lateral 

 branches towards the pinnulse, discovered by Roemer and Yandell, 

 most likely lay above the plates, not under them, and the am- 

 bulacra were probably covered by a soft membrane, as in Pen- 

 tacrinus. 



The general arrangement of the ambulacral vessels follows one 

 plan in all Echinoderms, but the composition of the ambulacral 

 skeleton and the position of the ambulacral vessels in relation to 

 it are subject to very great variations in the different organs. The 

 ambulacral plates of the Sea-urchins, Starfishes and Crinoids 

 differ essentially from one another, as much as the system of 

 dorsal and interambulacral plating. 



There exist however cei'tain azygos median pieces in different 

 divisions, which, when they are present, lie upon the dorsal side 

 of the ambulacral vessel ; to this series belong the subambu- 

 lacral plates of Pentacrinus, the great subambulacral plate of the 

 Pentremites, the rotulse in the oral skeleton of the Sea-urchins, 

 and those portions of the oral skeleton of the Holothuriada above 

 which the ambulacral canals pass to the walls of the body. 



Cystidece. — Among the Crinoids the Cystidea of L. von Buch 

 form a group which is distinguished by the inclusion of the ge- 

 nital organs, together with the other organs, in the calyx. In 

 the Pentacrinites and Comatula on the other hand, the sexual 

 organs are attached to the pinnulse of the arms ; in those Cri- 

 noids which have only one calycine opening (mouth), as Actinocri- 

 nus, Platycrinus, &c., the exclusion of the sexual organs from the 

 calyx is at once rendered probable by the absence of any aper- 

 ture corresponding with them. The Cystidece, on the other hand, 

 have at least two and sometimes three apertures to their calyx, 

 one of which, distinguished by its valvular closure, is found in 

 no other Crinoids than the Cystidece. L. von Buch has deter- 

 mined that this valvular pjTamid is the genital aperture. We 

 owe to him the recognition of the close alliance of these forms 



