306 Dr. T. Williams vn the Mechanism of Aquatic 



office of breathing. It intex'venes between the contents of the 

 digestive cseca and the aerating element. The link of zoological 

 continuity between the Echinodcrms and the Annelids is not 

 more clearly constituted by Aphrudita aculeata, than the Cestoid 

 and Trematode Entozoa are joined to the Annelids by the trans- 

 itional family of the Nemertinidee. From the Entozoa the latter 

 differ in the extraordinary feature of being embraced in ciliated 

 epidermis. Every part of the external surface of the body in the 

 Nemertinidse is the scene of active ciliary vibration. No ap- 

 proach to the development of this epithelium occurs in any En- 

 tozoon. The live genera Valencia, Borlasia, Nemertes, Polia 

 cerebratulus and Oerstedia, into which M. Quatrefages has distri- 

 buted the Nemcrtine Annelids, are exemplified on our coasts. 



From the Cestoid and Trematode Entozoa several of these An- 

 nelids are distinguished by the presence of corpuscles in the chyl- 

 aqueous fluid. In others of them the Huid conforms in character 

 to that already defined in these parenchymatous worms. To the 

 latter the Nemertinidse are united by another striking pecu- 

 liarity, that the ca?cal diverticula of the alimentary system are 

 filled with a corpusculated fluid, which, from the methodized dis- 

 tribution of these parts throughout the body, participates un- 

 questionably in the function of respiration. The Nemertinidse, 

 intimate though their alliance may be with the Cestoid and 

 Trematode Entozoa in general plan of construction, are separated 

 from the latter in one important particular : — in the Annelids 

 the blood system is obviously present, the blood-proper being 

 brightly red in colour ; in the Entozoa the existence of this 

 system must for the present be held as doubtful. 



It remains to consider the mode in which the process of 

 breathing is accomplished in the Abranchiate Annelids, i. e. the 

 leech, the earth-worm, and the Nais. 



In all systematic works these worms are summarily dismissed 

 as " breathing by the surface." In Nais filiformis (fig. 10) the 

 blood-proper is only very scantily distributed over the cutaneous 

 siu'face ; it is impossible therefore that it can be the imme- 

 diate subject of the first act of aeration. The visceral chamber 

 [d) in this little worm is filled with a corpusculated fluid : in 

 this fluid coils of blood-vessels (/, /) are suspended. The 

 blood-proper systematically, by expressly provided vessels, thus 

 brought into intimate contact with the chylaqueous fluid, inter- 

 changes constituents with the latter : the former yields up to 

 the latter its carbonic acid, and the latter to the former its 

 oxygen. The chylaqueous fluid thus becomes to the blood-pro- 

 per the aerating medium. Respiration thus explained is literally 

 internal, but not the less real. 

 In the instance of the earth-worm the (hylaqucous fluid is ahuos^t 



