the Crinoiclea, Cystidea, and Blastoidea. 259 



In a large liydrospire there are about twenty of tliose tubes ; 

 their greatest breadth is at their mid-length, where they are 

 crossed by tha suture (c c) 5 and as they become narrower 

 accordingly as their length decreases, the one in the middle 

 projects the deepest into the perivisceral cavity. In con- 

 sequence of this arrangement, when a section is made across 

 the hydrospire at the suture, c c, fig. 4 c/, the form h is obtained, 

 where c c is the surface of the shell, while the comb-like struc- 

 ture below represents the tubes. 



Specimens of G. ornatus almost entirely empty are often 

 found ; and in some of these the internal form of the hydro- 

 spires is sometimes preserved. Those that I have seen have 

 the form of small rhomboidal pyramids, with four slightly 

 convex sloping faces, and composed of a number of vertical 

 parallel plates (the casts of the interior of tlie tubes), the sub- 

 stance of the tube itself not being preserved. I have, how- 

 ever, several polished transverse sections in which I think the 

 thin walls can be seen. 



The structure of the hydrospires is such that there can 

 scarcely be any doubt that they are respiratory organs. The 

 sea-water entered through the pores, and aerated the chyla- 

 queous fluid contained in the perivisceral cavity by trans- 

 fusion through the exceedingly thin membranous shell that 

 composed the walls of the tubes. The number of pores varies 

 with the size of the individual. In large specimens there are 

 from 800 to 1000. 



It has been stated by some authors that the pores were pas- 

 sages 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 tubercles 

 (those with a vesicular and spongy summit) such protrusion 

 would be utterly impossible. 



In Caryocrinas 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 centre. 



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 



