Respiration in Invertebrate Animals. 183 



the larva of the Insect therefore bears a closer typal analogy to that 

 of the Annelid than that which is presented by that of the lowest 

 Myriapod. The dorsal vessel of certain transparent aquatic 

 larvae may be readily defined under the microscope; its pul- 

 sations and currents may be perfectly observed. Tlie corpuscles 

 floating in the blood mark with great clearness the direction of 

 the ciu-rent. It is certain that in some species the systemic 

 artei-ies (fig. 1,/, & fig. 3, n, n.) (of Mr. Newport) do not exist. 

 In those in which these vessels are detectable, the current which 

 they convey tends towards, not from, the dorsal vessel. Repeated 

 observations have convinced the author upon this point. They 

 are venous, not arterial ; they return the blood from the viscera 

 into the dorsal vessel ; these vessels are described everywhere, in 

 all his writings, by Mr. Newport, as visceral arteries; his dis- 

 sections have notwithstanding traced them into anastomosis with 

 branches coming from the supraspinal artery (fig. 1, c, fig. 3, c, e, 

 fig. 2, i). In this vessel the blood moves from the head in the 

 direction of the tail ; it follows therefore that the currents con- 

 veyed by the anastomosing branches must meet each other in the 

 same vessel ! action and reaction are equal and contrary ! stag- 

 nation results ! The subspinal venoxL^ trunk (fig. 3, e, I) discovered 

 by ^Ir. Newport in Insects and Arachnids does not exist in the 

 Myriapod. In the latter, therefore, the primary channels of the 

 system of the blood do not exceed two in number, the dorsal and 

 the ventral. This is the case in the Annelid ; the ventral vessel 

 of the Annelid undulates much less obviously than the dorsal. 

 It receives all its blood from the dorsal by means of the 

 oesophageal collar-branches : it distributes it chiefly to the 

 integumentary structures. Trunks of secondary size proceed 

 backwards, in some species from the heart, in others from the 

 oesophageal vessels, expressly to supply the walls of the alimentary 

 canal*. Now the veins which retui-n this blood from the glan- 

 dular parietes of the intestinal canal in the Annelid enter into 

 the dorsal vessel precisely in the same manner as the systemic 

 arteries are described by Mr. Newport to proceed from this vessel 

 in the Myriapod, Insect, and the Arachnid. In classes so conti- 

 guous, why should the functions of the same vessels be reversed ? 

 Mr. Newport's views are drawn from anatomical structures ; he 

 has never seen the blood moving in these so-called systemic 

 arteries ; he ignores the argument of analogy drawn from the 

 living circulation of the Annelid; he does not perceive the me- 

 chanical diflSculty with which the blood would enter these vessels 

 from the segmental heart, on the supposition of their arterial 

 character. They arise from the latter at its extreme posterior 



* See the Author's Report on the British Annelida in the Transactions 

 of the British Association for 1851. 



