34b Dr. T. Williams on the Mechanism of Aquatic 



Nemertine orders (Plate XIII. figs. 1, 2) will be afterwards 

 shown to fall under this categorj'. In Aphrodita aculeata (fig. ^,d) 

 the digestive diverticula are filled with a corpusculated fluid, 

 which is exposed by an express contrivance to the agency of 

 the surrounding aerating medium. This is also the case in 

 several species of freshwater planariform leeches (fig. 4). It 

 prevails too in the true marine Planariae (fig. 3). To the 

 value which the author has endeavoured to assign to the fluid 

 systems of the Cestoid and Trematoid Entozoa, no sound analogy 

 is opposed. These worms are, on the ground of the interpreta- 

 tion of the fluids now first offered, naturally linked into a continu- 

 ous zoological chain with the lowest parenchymatous Annelids. 

 Notwithstanding the meritorious researches of M. Blanchard, 

 the organization of the Cestoidea is even now a theme prolific of 

 controversy. The large, branched, flocculent organ (fig. 6) form- 

 ing the bulk of each segment in Tania and Bothriocephalus is 

 designated by all recent writers after M. Blanchard as the ova- 

 rian apparatus. It is really the alimentary organ. It opens ex- 

 ternally by an orifice proper to each segment. This organ in 

 each segment is therefore independent. It sucks nourishment 

 from without by its own separate mouth. It is a Planaria in 

 itself. 



What are ordinarily described as the straight canals along either 

 edge of the body are not gastric but chylaqueous. They ai-e a 

 part of a system of open channels ramifying through the cells 

 of the parenchyma in which the gastric caeca are lodged. The 

 latter are almost surrounded externally by the fluid filling the 

 former. This fluid, as formerly explained, although non-cor- 

 pusculated, constitutes the real chylaqueous system of the Cestoid 

 and Trematoid Entozoa. It is through its agency that the 

 breathing function is chiefly accomplished. 



In Teenia this fluid system is' common to the whole body of 

 the animal. The ' segments ' therefore are separate units only 

 as respects the alimentary and reproductive organs. The chyl- 

 aqueous fluid attains its enclosing channels by exosmose from the 

 alimentary organ, not directly ab extra. The posture of the ani- 

 mal in its native habitat favom-s this interpretation of its organism. 

 The orifice of each segment is applied to the surface of the infested 

 part. The necessity for the preceding explanation illusti'ates the 

 intimacy with which the respiratory function is interwoven with 

 all the other nutritive operations of the body. In the TcBuice then 

 the respiration is cutaneous, but not in the mode commonly sup- 

 posed. The skin is not the scene of a rich plexus of true blood- 

 vessels. It is permeated and pervaded evei-ywhere by that fluid 

 which embraces the alimentary organ, and which is distributed 



