STRUCTURE OF THE CRINOIDEiE. 15 



sides like a fence of minute palings. These are the " margi7ial ^plates 

 of the ambulacral grooves.''^ 



Figure 1. 



Figure 1. Diagram of tlie ventral surface of Pentacrinus caput-Medtisce. The 

 central orifice is supposed to be the mouth ; the other, the anus. One of 

 the grooves is represented as being closed over by the marginal plates. 



The grooves are covered passages, along which are conveyed from 

 the interior of the body to the arms and pinnulae a number of 

 tubular vessels whose functions appear to be of great importance in 

 the physiology of the Crinoids. As the eggs from which the young 

 are produced are developed in the pinnulae, no doubt there must be 

 an organ of some kind connected with their generation which 

 communicates with the viscera of the animal by passing along the 

 grooves. Another set of vessels are the aquiferous canals, consisting 

 of long, slender tubes, for the conveyance of the fluid by which the 

 sucking feet of the arms and pinnulge are extended or retracted. 

 To these must be added the blood-vessels, nervous filaments, and 

 muscles. Traces only of these have been actually observed, but the 

 almost perfect identity in structure between the ambulacra of the 

 Crinoids and those of the Star-fishes, in which it is well known that 

 such organs do exist, renders it quite certain that the former as well 

 as the latter are provided with a full set of ambulacral vessels. 



In many of the extinct species of Crinoids, although the arms and 

 pinnulae are grooved, yet there are no grooves leading from the 

 bases of the arms to the mouth ; and it therefore becomes probable 

 that the ambulacral vessels of the arms and pinnulae do not enter 

 the body through that orifice. Indeed in a great many species, as 

 the mouth is situated in the top of a tube which is sometimes longer 

 than the arms and rises above them, it seems impossible that they 



