Respiration in Invertebrate Animals. 259 



adding the confession, that "die Injection vom Blutgefassring 

 gelang nur his zum Anfang dieser Gefdsse." And this is all that 

 the author of this memoir has ever been able to accomplish to 

 prove the existence only of the central trunks. This is the sum 

 of the existing knowledge with reference to the blood-proper 

 system of the Asteridse. Of that of the Echinidse very little also 

 is known. Miiller describes a circular vessel embracing the 

 oesophagus immediately underneath the lantern; from this a 

 trunk proceeds coursing along the curves of the intestine, and 

 ending in the circulus analis. This learned anatomist observes, 

 " Um eine klare Vorstellung vom Hersen zu bekommen, muss 

 man es bei Cidaris untersuchen ; es ist bei Cidaris ein weiter, 

 ganz gerader Canal mit dicken weichen Wdnden *." In Cidaris 

 the heart is not a circular vessel, but a fusiform trunk lying pa- 

 rallel with the intestine. The circumference of the blood-system 

 in the preceding genera has never yet been brought under de- 

 monstration. In science negative is inferior in value to positive 

 proof; but at present it is only possible to declare that no care 

 can succeed in discovering any evidence whatever of the presence 

 of a blood-proper system in the solid structures of any part of 

 the body of the Echinoderm. The parietes of the alimentary 

 canal are most certainhj not ramified by blood-vessels. The soft 

 parts of the integumentary system are literally destitute of vas- 

 cular tissue. The contents of the central trunks of the blood- 

 system are identical in appearance with those of the visceral ca- 

 vity, and with those of water-vascular or suctorial system ; that 

 is, when the morphous elements of these three fluids are placed 

 in juxtaposition under the microscope, it is impossible to indi- 

 cate between them any difference whatever in structure or shape. 

 The fluids themselves are also identical in every physical appear- 

 ance. 



The blood-vessels are internally and externally lined with cilia, 

 the water-vessels are so, and the visceral cavity is richly so. 

 T^rhat can these extraordinary facts mean ? Can they mean any 

 thing, but that these three systems are reciprocally coimected ? 

 A suspicion to this effect has been expressed by Milne-Edwards 

 and M. Quatrefages ; but the author claims the merit of having 

 first produced demonstrative facts which impart to this suspicion 

 a very probable character. The question admits of more confident 

 answer in the higher genera. The blood-vessel of the Sipuncle 

 may be readily exposed : it lies on the intestine in form a bright 

 pink thread (fig. 10, m,j) ; it exists only on one side ; it has no 

 discoverable correlate on the opposed side of the cylinder ; it is 

 filled with pink fluid, the corpuscles of which are identical un- 



* Op. cit. p. 127. 



18* 



