the Crinoitlea, Cystidea, and Blastoidea. 



261 



through the pores and filled the tubes, to be aerated through 

 the thin external covering by the surrounding water. In 

 Caryocrinus the water passed inward, through the pores, into 

 tlie tubes, ancf aerated the fluid within the general cavity of 

 the body. 



The discovery that the fissures and pores of the Cystidea do 

 not communicate directly with the general cavity of the body 

 is entirely due to Mr. Rofe. After reading his highly impor- 

 tant pa])er, I re-examined a great number of specimens, and 

 found sufficient to confirm his observations. 



3. On the Genus Codaster. 



Every author who has described a species of this genus lias 

 remarked the peculiar striated areas in the interradial spaces. 

 Prof. M'Coy, the founder of the genus, pointed out their re- 

 semblance to tlie hydrospires of the Cystidea ; but it was Mr. 

 Rofe who first showed that they were also identical in struc- 

 ture therewith. On comparing one of these with that of the 

 Cystidean Pleurocystites (fig. 5), we at once perceive that they 

 are the same in external form, while Mr. Rofe's figures show 

 that the section at ^fZ(fig.8) has the structure of fig. 9, which 

 only differs from fig. 5 h in being straight above instead of 

 concave, and in being divided into two parts. This division 

 is the result of the position of the arm, which cuts the hydro- 

 spire in two in a direction parallel to the fissures. By draw- 

 ing the j)oints d a and a d together, we get figure 10, which 

 is, in general plan, a section across one of the ambulacra of a 

 Pentremite. On examining nearly all the published figures 

 of s]')ecies of this genus, I find that there is a series of forms 

 wliich exhibit a gradual passage, from those w^ith the hydro- 

 spires almost entirely exposed (as in fig. 8), through others, in 



Fi^. 8. 



Fio;. 9. 



d 



Fig. 10. 



^ 



%^1^ 



Fig. 8. Summit of C. acutus, M'Coy : wi r, mouth and vent; d d, sutiu-e 

 acros.1 the posterior hyclrospire. Fig. 9. Section across the hydrospire 

 from d to d; at a is the place of the arm. Fig. 10. The section con- 

 tracted as in fig. G. Fig. 11. Summit of Peniremifcs caryoph/llattis, 

 De Koninck. 



