98 



perivisceral cavity, by transfusion through the exceedingly thin mem- 

 branous shell, that composed the walls of the tubes. ^ The number of 

 pores varies with the size of the individual. In large specimens these are 

 from 800 to 1000. 



It has been stated by some authors that the pores were passages for 

 the protrusion of internal organs connected with the vitality of the animal. 

 The fact, however, that the pores do not penetrate into the general cavity 

 of the body disproves this theory ; and, moreover, through many of the 

 tubercle s-r- those with a vesicular and spongy summit, such protrusion 

 would be utterly impossible. 



In Caryocrinus ornatus there are thirty hydrospires arranged as 

 follows : 



1. Ten at the base — half of each on a basal plate and the other half on 

 one of the subradials, their longer diagonal vertical. 



2. A zone of six around the fossil at the mid-height — their longer 

 diagonals horizontal. These seem to be imperfectly developed, for, on 

 the inside, the tubes occupy only a small space in the center. 



3. A third band of fourteen — two of them with their longer diagonals 

 vertical and the others arranged in six pairs, the diagonals of each pair 

 inclining toward each other, upward, at an angle of about 30*^. There 

 are only three interradii in Coryocrinus ; the mouth is placed in one of 

 them and the two hydrospires with vertical diagonals in the other two. 



In Tlewrocystites the hydrospires are also of a rhomboidal form, but 

 instead of having the tubular structure of CaryocriJius, they consist of a 

 number of parallel inward folds of an exceedingly thin part of the shell. 

 These folds no doubt represent the tubes of Caryocrinus. If we grind 

 down a hydrospire of this latter, so as to remove all the shell, and expose 

 the edges of the tubes, it then exhibits precisely the same form as fig. 54 



h e., the form of a rhomb, longitudinally striated at right angles to the 

 suture, and with no pores. The transverse section in Pleurocystites only 

 differs from that in Caryocrinus in having no shell between the points 



c. In the hydrospire of Pleura cijHites robustus, of the Trenton lime- 

 stone, we have the commencement of the formation of an internal gill 

 with a single spiracle. The surface is not flat, as it is in many species, 

 but concave as shown in the section ; and it is evident that if the conca- 

 vity should be carried further, and at the same time the points <?, c, made 

 to approach each other, the effect would be to produce an elongated sack, 

 deeply folded on one side, and with a fissure extending the whole length 

 on the other side. The transverse section of such a sack would be fig. 

 55, the same as in Pentremites. Again, if we contract the four sides, 

 gradually curving them outward at the same time, but not diminishing the 



