Respiration in Invertebrate Animals. 405 



which are readily observed on external examination (B, a). No 

 cilia exist on the interior surface : they are abundant and vigorous 

 on the exterior. A rich vermilion coil [b, c) filling the hollow of 

 the process and floating in tlie chylaqueous fluid may be seen by 

 transmitted light. This is a tiaie blood-vessel: it is a single 

 vessel. It is a law in many Annelids that the ultimate blood- 

 vessels do not form plexuses : this proceeds from the extreme 

 mobility of the body. The quantity of the blood-proper varies 

 as the Annelidan organism varies at different seasons; it is greatest 

 during the reproductive season, a season during which the chyl- 

 aqueous fluid is most reduced in amount. The two fluids^ though 

 coordinate in physiological capacities, are governed by inverse 

 laws. 



In Cirrhatulus Lamarckii and in the allied genus Ophelia, a 

 linear series of yellowish blood-red threads, remarkably irritable 

 and contractile, project to a considerable distance from either 

 side of the body throughout its whole length : at the occiput 

 they are grouped over the dorsum. They convey the blood- 

 proper exclusivelij in a single vessel of considerable length. 



The Aphroditacece constitute a group of Annelids to which the 

 term dorsibranchiate by no means correctly applies ; that is, in 

 the majority of the species embraced in this order no branchial 

 appendages exist either on the dorsum or sides. In all the 

 Ap/iroditacece the blood is colourless. The blood-system is in 

 abeyance, while that of the chylaqueous fluid is exaggerated. 

 But it is exaggerated only in bulk ; it is not raised in organic 

 composition ; its corpuscles are scanty, and its albumen small in 

 relative amount. This unusual fact is explained by the presence 

 of organized corpuscles in the dark chymous fluid which fills the 

 gastric diverticula. The scales or elytra fulfil an important 

 purpose : they rise and fall. In rising under muscular action, 

 they create a vacuum in the space between them and the back, 

 into which the water rushes ; in falling or collapsing, the water 

 escapes in a current posteriorly. These currents of water operate 

 immediately upon the fluid contained in the gastric pouches. The 

 latter are arranged so as most advantageously to receive the 

 influence of the external aerating element. But they float also 

 in the chylaqueous fluid : tliis is also in part oxygenized. It 

 is the agent by which this vivifying element is conveyed to the 

 solids of the body ; it shares directly in the function of respi- 

 ration; it receives its organic principles from the contents of 

 the gastric caeca. 



It cannot have escaped observation, that there prevails a 

 striking resemblance between the general anatomy of Aphrodita 

 aculeata and that of the Asteridae among the Echinoderms. In 

 the latter, however, the chylaqueous fluid fulfils exclusively the 



