the Criiioidoa, Cystidca, (dkI Blastoidea. 



201 



tliroii^li tlie })orcs and filled the tubes, to be ai'rated tlirouu-li 

 the tliiii external covering by the siirr(^undin<^ water. In 

 Can/ocrinus the water passed inward, through the pores, into 

 the tubes, and aerated the fluid within the general cavity of 

 the body. 



The discovery that the fissures and pores of tlie Cystidea do 

 not eoninHinicate directly with the general cavity of the body 

 is entirely due to Mr. Kofc. After reading his highly ini|)or- 

 tant j)a))er, I re-cxaniined 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 has 

 remarked the peculiar striated areas in the intcn-adial spaces. 

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

 semblance to the 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 P/ewocj/sfitrs (fig. 5), we at once perceive that they 

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

 that the section at dd{i\g.8) has the stmeture of fig. 9, whicli 

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

 concave, and in being div'ided into two parts. Tliis 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 points d a and a d together, we get figure 10, which 

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

 Pentremitc. On examining nearly all the published figures 

 of s))ecies of this genus, I find that there is a scries of forms 

 whieli exhibit a gradual passage, from those with the hydro- 

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



Fig. 8. Fig. 9. Fig. 11. 



dad 



Fig. 10. 



CS~^fi^^ 



Fig. 8. Summit of C. acutm, M'Coy : wj r, mouth and vent; d d, suture 

 across the posterior hydrospire. Fig. 9. Section across the hydrospire 

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

 tracted as in fig. 6. Fig. 11. Summit of Pentremites canjophijUatus, 

 De Koninck. 



