310 Dr. T. Williams on the Mechanism of Aquatic 



distributed over the exterior of the vessels of the gills (PI. XVIII. 



The part enacted by the adipose element in the physiological 

 act of respiration, replete with novel interest, belongs to the 

 physiological bearing of the question of respiration*. 



In the preceding review of the mechanical condition of the 

 respiratory process as it occurs in the higher Articulata, the 

 physiologist must have observed the comparative fewness of the 

 corpuscles of the blood (PI. XVIII. fig. 11), the smallness of the 

 bulk of the blood relatively to the dynamical capabilities of the 

 articulated animal, the small proportion of blood which in a 

 given time traverses the branchial organs, the breadth of the 

 ultimate blood-stream measured by the size of the pulmonary 

 capillary of the vertebrated animal- These facts seem certainly 

 to prove that there can exist no direct proportionality between 

 the amount of respiratory process and the general dynamical 

 capabilities of the Crustaceaf. 



EXPLANATION OF PLATES XVIL and XVIIL 

 Plate XVH. 



Fig. 1. Appendages of the ninth pair composing the external foot-jaws of 

 Decapod Crustaceans, and which are homologous with the sub- 

 cyhndriform process of the first pair in Squilla, with the thoracic 

 feet of the twelfth in Isopodan Crustacea, and with the thoracic 

 feet of the third pair in the Branchiopoda : a, internal element ; 

 b, palp ; d, branchia ; c, flabellum. 



Fig. 2. Appendages of the eighth pair constituting the foot-jaws in the 

 Decapoda,— which are homologous with the pr*'hensile jaws in 

 Squilla, with the thoracic feet of the first pair in the Isopods, 

 and with the branchial processes of the second pair in the 

 Branchiopods. 



Fig. 3. Abdominal appendage of Squilla : a, internal member; d, external; 

 b, branchia. 



Fig. 4. Abdominal appendage of the Isopoda : a, external or cutaneous 

 process : b, internal and branchial. 



Fig. 5. Abdominal appendage of a Branchiopod. The dotted process is 

 the respiratory. 



Fig. 6, A. Caprella linearis {male) : a, respiratory appendages ; b, the same 

 still further magnified ; 6', shows the single blood-current moving 



* See ante, " Process of Respiration." 



t I have understood that some years ago a paper was read by Prof. 

 Quekett, before the Microscopic Society of London, " On the structure of 

 the flahella in the Crustacea." It has never been my good fortune to see 

 that paper. The description given in the text is founded upon a very 

 extensive series of original examinations. I am desirous here to pay the 

 tribute of my gratitude and admiration to Prof. Milne-Edwards, for the 

 assistance and instruction which I have received from the study of his 

 numerous splendid contributions to this branch of comparative anatomy. 



