358 Dr. T. Williams on the Mechanism of Aquatic 



imperfectly formed corpuscles always the same in the same 

 species. 



In other species of Asteridae the membranous appendages of 

 the skin present other varieties in size and figure, none in cha- 

 racter and structure. 



In Uraster papposa the membranous intervals in the calcareous 

 trellis are large and favourable for examination. They may be 

 readily seen with the naked eye in the living animal to be capable 

 of being bulged out under the pressure of the fluid in the visceral 

 chamber. 



The Cribellidae present another modification of the soft part 

 of the integument. The skin is smoother, the membranous in- 

 tervals are smaller, and the membranes are less capable of pro- 

 trusion outwards. In every other respect they are identical with 

 the corresponding parts of the preceding genera. The propo- 

 sition may now then be finally affirmed, that in the Echino- 

 dermata the chylaqueous fluid [i. e. the contents of the visceral ca- 

 vity) is itself first aerated, and that by means of a machinery of 

 soft pai'ts expressly arranged with a view to this end ; and that 

 then it aerates the blood-proper. 



3. Water-vascular and Blood-vascular Systems of the Echinoderms. 

 To what extent, in what manner, if at all, do these two fluid- 

 systems, or either of them, participate in the mechanism of the 

 respiratory process ? In solving the curious problem presented 

 by the Echinodermal organism, the highest interest attaches to 

 this question. Let it be first seen whether the statements of 

 Tiedemann, Sharpey and Miiller, that these systems are per- 

 fectly independent of each other and of the chylaqueous system 

 already described, are really founded on trustworthy demon- 

 stration. No anatomist up to the present time has done more 

 towards elucidating the anatomy of the true blood-system of the 

 Echinoderms than that of proving the existence of certain central 

 trunks only. This system has never been traced to its peripheric 

 distribution in any species (the Holothuridan perhaps excepted) 

 by any comparative anatomist. An induction is unsafe which is 

 grounded upon hypothesis. The theory of respiration can only 

 be constructed out of the materials supplied by the patient 

 labours of the anatomist. It does not appear that Dr. Sharpey 

 ever could verify the description of Tiedemann with reference to 

 the blood-system. The tenuity of their coats however, and pale 

 colour of their contents, render it extremely difficult to trace 

 completely the distribution of the vessels*. Miiller concurs in 

 the description of Tiedemann, stating that in Asterias a circular 

 trunk surrounds the mouth and gives ofi" branches to each ray — 

 * Art. Echinoderinata, by Dr. Sharpey. 



