132 Dr. T. Williams jon the Mechanism of Aquatic 



this particular link in the chain, are these events declared? 

 How are they to be explained ? Is it necessary in the scheme of 

 creation that the Annelid should chronologically antecede the 

 Myriapod ? Are the fluids of the Annelid plus fibrine suddenly 

 invested with such new building capacity as to be enabled to 

 construct a Myriapod? The sera will assuredly arrive when 

 organic science will satisfactorily answer such transcendental 

 questions ; there repose, beneath the curtain of the theories of 

 spontaneous generation, specific transmutation, progressive deve- 

 lopment, &c., truths moi'e recondite than any yet projected by 

 the genius of the author of ' The Vestiges,' &c., or defended by 

 the ingenuity of his countless reviewers. 



The Myriapod is the lowest articulate animal, the Annelid 

 the highest annulose: though constituting juxtaposed classes, 

 they are yet divided by deep differential characters. The circu- 

 lating system of the articulated animal is distinguished by one 

 remarkable fact : only the central (dorsal) vessel enjoys the power 

 of contracting and dilating ; every other part of the circulatory 

 apparatus is passive. The supra-spinal vessel in the Myriapod 

 and the insect is not pulsatile ; it is like the abdominal aorta in 

 fishes. It is separated from the contractile centre by the inter- 

 vention of narrow branches, the aortic arches, which embrace the 

 oesophagus. The pulse-wave imparted to the fluid current by 

 the ventricular action of the dorsal vessel is broken by these 

 straitened tubes. The system of the branchial capillaries in the 

 fish converts the saltatory manner in which the blood moves in 

 the interval between them and the heart into a continuous non- 

 pulsatile current. Thus the velocity of the current, and the 

 force with which it travels, are reduced. A slackened course is 

 impressed upon the blood -stream in every part of the body. 

 From these anatomical facts will hereafter flow physiological 

 consequences of great importance ; they will unriddle the arcana 

 of the second stage of respiration. The parietes of the peripheral 

 channels, though undoubtedly constituting independent mem- 

 branes, are adhei-ent externally to the solid structures amid which 

 they penetrate. In this particular they difi^er strikingly from the 

 corresponding parts of the true-blood system of the Annelid. 

 In the latter case, every vessel, the minutest and the largest, is 

 detached from all other structures, appearing everywhere in form 

 of independent systolising and diastolising tubes. 



The dorsal vessel of the articulate animal is much more perfect, 

 viewed as an hydraulic instrument of propulsion, than that of 

 the Annelid. In a few species of Annelids, indeed, a cordi- 

 form development of this vessel occurs ; it is, notwithstanding, 

 little distinguished from the rest of th^ apparatus ; centralization 

 is not required. Every segment of the system, periphery and 



