244 Dr. T. Williams on the Mechanism of Aquatic 



aerated, the less involved is the disposition of the solid parts 

 through which the exposure is effected. Elaborately vitalized 

 blood is circulated through respiratory organs of inconceivable 

 complexity and subdivision. The study of the mechanical 

 conditions of respiration should regard the fluids as well as the 

 solids of the structures dedicated to this function. The floating 

 cells of the fluids are concerned in, though not essential to the 

 respiratory process. The true capillary segments of the lungs of 

 all vertebrated animals, those of reptiles not excepted, are destitute 

 of vibratile cilia. The branchiae of fishes, without a single known 

 exception, are clad only by a won-vibratile epithelium. The 

 general proposition, that ciliary epithelium constitutes no part 

 of the active portions of the breathing organs of vertebrated 

 animals, rests therefore upon the stable basis of actual demonstra- 

 tion. This negative must be changed into an affirmative state- 

 ment, with reference to the air-passages, which in the pulmonary 

 vertebrated series are profusely and universally ciliated. The pre- 

 sence of cilia on the branchial structures of invertebrated animals 

 is a common, but not a constant fact ; the rule without intelligible 

 reason is suspended in numerous instances. The operation of 

 cilia is therefore not indispensable to the respiratory process, 

 even in the invertebrate animal — not an essential constituent even 

 of the aquatic model. The blood of all vertebrated animals is 

 richly charged with corpuscular elements. It is invariably 

 coloured red. In the invertebrated subkingdom not one example 

 is known of a corpusculated red-hlood. The blood of every 

 known mollusc bears floating solid elements. In every arti- 

 culated animal the true-blood abounds in organized corpuscles. 

 In every annelid, without a single known exception, the blood- 

 proper is perfectly destitute of morphotic elements ; it contains 

 no trace of visible cells. The perfect fluidity of the true-blood of 

 all Entozoa can also now be affirmed. Every Echinoderm is en- 

 dowed with an imperfect blood-system, the blood-proper bearing 

 cells in suspension. The chylaqueous fluid of every animal in 

 which it exists, is charged more or less abundantly with organized 

 corpuscles. The generalization is thus incontrovertibly esta- 

 blished, that there exists no single instance of a real animal, of 

 which one or other of the circulating nutritive fluids of the 

 organism, is not replete with morphous particles. What office, 

 if any, these floating solids exercise in the mechanism of the 

 respiratory act, it is the province of the physiologist to determine; 

 their microscopic characters it is the duty of the anatomist to 

 describe. 



Porifera. — In the Spongiadse, the fluids to be aerated are 

 contained in and between the component cells of the gelatinous 

 cortex. Each separate cell, like that of Amoeba, is an inde- 



