THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[SECOND SERIES.] 

 No. 111. MARCH 1857. 



XV. — On the Mechanism of Aquatic Respiration, and on the 

 Structure of the Organs of Breathing in Invertebrated Animals. 

 By Thomas Williams, M.D., F.L.S., Physician to the 

 Swansea Intirmary. 



[Concluded from vol. xvii. p. 258.] 



[With a Plate.] 



Cephalopoda^. 



The Cephalopod Mollusks stand at the head of the Invertebrated 

 series, not only in virtue of a superiority of bodily form, but 

 also of the higher type of structure which pervades their entire 

 organization. They depart from the Gasteropods in the position 

 and shape of their principal prehensile and locomotive organs, 

 and in the significant fact that in them the ^general cavity' of the 

 body is almost obliterated. They approach the Verteb rated animal 

 in the following respects: — They possess a rudimentary endo- 

 skeleton, greatly developed cephalic ganglions, large and active 

 organs of sense, a vigorous and well-formed muscular system, a 

 blood-vascular apparatus more perfect than that of any other 

 class of Invertebrata, a nutritive fluid thickly fibrinized and of 

 high specific gravity, and blood-corpuscles which in figure and 

 structure more nearly resemble those of the Mammal than do 

 those of any other Invertebrated animal. The circulatory appa- 

 ratus of the Cephalopods does not, however, constitute a perfectly 

 closed system. This point was established by the early classic 

 researches of Professor Owen. The cephalic venous sinus, and 



* The author regrets that, in consequence of various engagements, he 

 lias been obliged to postpone so long the publication of this last paper, 

 which completes the series published in the 'Annals' under the above title 

 from time to time during the years 1854-1856. 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. xix. . 13 



