184 Dr. T. Williams on the Mechanism of Aquatic 



end (PI. IX. fig. 3, n, n) — at that very point at which they should 

 enter if they were veins. 



A large wave of blood rnshing forwards may be clearly seen in 

 the dorsal vessel of larvae (fig. 4^ h, a) : the chambered dorsal 

 vessel contracts from behind forward ; it observes the law of the 

 Annelidan ; the hindmost chamber contracts first, that next to 

 it in advance next, and so on. The systolic movement does not 

 occur throughout the whole length of the vessel at the same 

 time, but in parts from behind forwards as the wave of blood 

 travels. In two adjacent chambers the actions of systole and 

 diastole are alternate. 



There is in fact no functional difi^erence whatever between the 

 dorsal vessel of the Annelid and that of the articulated animal ; 

 they ai*e distinguished only in structure. In the Myriapod, the 

 tubular vessel of the Annelid is reinforced with propelling power 

 at successive points = the chambered hearts. In the articulated 

 animal this extra power in the central vessel is absolutely re- 

 quired. In the Annelid every vessel in the body is an elastic 

 tube tightly embracing the fluid contents. The channels through 

 which the blood moves in the Ai'ticulata do not closely grasp the 

 contained fluid (fig. 2, g) ; they are indeed bounded by definitive 

 membranes, but they are not elastic contractile structures fitted 

 mechanically to favour the circulation of the fluids (Pl.X. fig. 10, 

 b, c). This circumstance ought not to involve an alteration in the 

 tgpe of the circulation. But if the systemic arteries of j\Ir. Newport 

 be really arteries, then the plan and principle of the circulatory 

 systems of the Annelid and the Myriapod must be diametrically 

 dissimilar. If they be arteries, the circuit of the fluids cannot 

 be explained without involving physical contradictions. The 

 author is however convinced that Mr. Newport has committed 

 the mistake of imputing to vessels an arterial character, which 

 observation and analogy prove to be venous. 



The orbit of the blood-current in the Myriapod conforms in 

 every particular with the Annelidan type. All vessels attached 

 to the dorsal vessel behind the oesophageal collar (fig. 1, b) are 

 afferent with respect to that trunk. All branches connected 

 with the great ventral or supraspinal vessel posterior to the same 

 limit ai'e efferent with respect to this vessel. The currents in 

 these latter branches are divisible into two orders — that first 

 which supplies the viscera, that secondly which is distributed to 

 the integuments : the current from the first, after having tra- 

 versed the viscera en route, returns into the dorsal vessel under 

 a venous character by means of the systemic arteries (sic) of Mr. 

 Newport. That from the second is poured from various sources 

 into the great splanchnic sinuses which enter the dorsal vessel 

 at the auricular orifices. 



