On the Mechanism of Aquatic Respiration. 393 



XXXVIL — On the Mechanism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

 By Thomas Williams, M.D. Lond., Licentiate of the Royal 

 College of Physicians, formerly Demonstrator on Structural 

 Anatomy at Guy's Hospital, and now of Swansea. 



fWith a Plate.] 

 [Continued from p. 348.] 



Annelida. — In the Annelida the function of respiration is dis- 

 charged under two remarkably distinct conditions. Under the 

 first, the chylaqueous fluid alone is subjected to this process; 

 under the second, the blood-proper exclusively fulfils the office. 

 The mechanical organs subservient to this function under the 

 former, are constructed on a plan diametrically difi"crent from 

 that of those provided under the latter circumstances. In the 

 Annelid the true-blood and chylaqueous fluid, though coexistent 

 in the same organism, constitute two perfectly distinct and in- 

 dependent fluid systems. There is between them no direct 

 communication of any sort ; they are physically veiy dissimilar 

 fluids. An order of branchial processes, intermediate between 

 the two preceding, must also be recognised, in which in equal or 

 unequal proportions the chylaqueous fluid and the blood-proper, 

 either in the same or in distinct appendages, participate in the 

 process of respiration. The branchial appendages affect four 

 different situations on the body: 1. on the head; 2. along the 

 back ; 3. along the sides, and 4. at the tail. The first rank 

 under theCephalobranchiata, the two succeeding under theDorsi- 

 branchiata, the fourth are represented by the Clymenida. The 

 Abranchiate Annelida resolve themselves also into two distinct 

 divisions ; those, first, which breathe through the agency of the 

 chylaqueous fluid, and those, secondly, which expose the true 

 blood. Both these groups would be comprehended under the 

 Cryptobranchiata of Dumeril. 



To these extremes, too, an intermediate order occm-s ; it em- 

 braces the NemertinidcE, the Liniadce and the Gordiitsida, the 

 cutaneous external surface of which is wonderfully and richly 

 ciliated. In these unfamiliar genera the chylaqueous fluid and 

 the true blood share, in unequal measure, however, the office of 

 appropriating oxygen. Thus in sxiccinct language has been de- 

 fined " the heads " imder which, in this interesting class, the 

 mechanical conditions of respiration must be studied bv the 

 anatomist. The breathing is accomplished in every species, the 

 earth-worm not excepted, in strict conformity with the aquatic 

 principle. No known Annelid respires on the atmospheric 

 model. In every Annelid the blood, though variable in colour, 



