256 Dr. T. Williams on the Mechanism of Aquatic 



structures. They are not perforated at their extremities (b) . They 

 cannot therefore serve as open passages for the direct admission 

 of the external medium into the visceral cavity. They are un- 

 questionably in part tactile appendages. In addition to the me- 

 ridional rows of suctorial feet, the shell of Echinus is perforated 

 by numerous hollow membranous processes (fig. ^,f,f,f), lined 

 within and without by vibratile cilia, and penetrated exclusively by 

 the fluid of the visceral cavity. Like the integumentary struc- 

 tures of the higher genera, they bear no evidence whatever of 

 blood-vessels. These facts impel the physiologist to the adoption 

 of one inference. They can only subserve a respiratory purpose on 

 the supposition that the subject of that process is the chylaqueous 

 fluid. Then the conclusion cannot be evaded, that, although in 

 the Echinidce the fluid contained in the visceral cavity may look 

 hke simple sea water, it must be something more ; else nothing 

 would be signified by the express provisions supplied, to subject 

 it to the process of aeration. It is, in truth a dilute albuminous 

 solution, charged with corpuscles indeterminately organized. It 

 possesses a higher solvent power for oxygen than simple sea 

 water. It is the reservoir out of which the elements of the true 

 blood are drawn. Injection thrown into the hollow of the shell 

 of Echinus distends beautifully, in relief, numerous membranous 

 appendages belonging to the integumentary system. In no in~ 

 stance whatever can zxxy perforations in the extremities of these 

 processes be detected. There is therefore no direct evidence for 

 the opinion commonly entertained by the best observers of the 

 Echinoderras, which affirms that the external water enters imme- 

 diately through openings in the integuments into the peritoneal 

 cavity*. 



The preceding questions admit of more easy and satisfactory 

 solution in the Asteridse than in the classes of Echinoderms 

 already reviewed. Asterias rubens is a large animal ; the fluid 



* On the subject discussed in the text, Miiller, in his recent elaborate 

 essays on the Echinoderms, pubUshed in his ' Archiv,' offers the following 

 remark, which I produce in the original : — " Die baumformigen Kiemen der 

 Seeigel, die ausseren Kiemen Valentin's soUen an den Enden ihrer Aeste 

 nach Tiedemann offen sein, auch das Wasser in das Innere des Seeigels auf- 

 nehmen. Diese Ansicht griindet sich auf Injection mit Quecksilber unter 

 gleichzeitiger Anwendung von gelindem Druck. Valentin fand diese 

 Organe dagegen an den peripherischen Enden geschlossen, so dass sie also 

 hohle Verlangerungen der Leibeshohle nach aussen darstellen, und die 

 Athemfunction auf ilirer aussern Oberflache stattfindet. Auch ich habe 

 bei wiederholter microskopischer Untersuchung dieser Theile von lebenden 

 Seeigeln keine Oeffuungen an den abgerundeten Enden wahrgenomraen." 

 Miiller, however, does not explain in what manner the " Athemfunction " 

 of these appendages, which he argues to be cajcal, is possible. . His conclu- 

 sions as to the imperforate character of these parts I have repeatedly con- 

 firmed by variously devised methods of examination. 



