Respiration in Invertebrate Animals. 411 



defined as maintaining its individuality from the beginning to 

 the end of its course. This fact identifies the branchiae of the 

 Chitonidse with the ]\Iolluscan type, and severs them from the 

 Crustacean. This is an anatomical character of essential im- 

 portance*. 



Although an ultimate leaflet from the gill of the Chiton has 

 precisely the same figure as the corresponding part of the gill 

 of a brachyurous Crustacean, its real structure is demonstra- 

 tively Molluscan. The microscope is thus enabled to prove, that 

 beneath an exterior general resemblance there lies hidden an 

 essential identity of organization. In the Chiton, as in all moU 

 lusks, the branchial vessels are individualized channels bounded 

 by distinct walls. In the Crustacea the blood traverses wall-less 

 lacunose passages, and forms invariably only a single stratum in 

 its course. These facts are beyond question. They prove that 

 the Chitonidse are far more intimately allied to the INJoUusk than 

 to the Crustacean. They establish a new principle in homology. 

 They prove that conformity in the last elements of structure 

 signifies more than the superficial analogies of outward form. 



Another striking point of dissimilitude between the gill of the 

 Chiton and that of the Crab, is that in the former the whole 

 lamina, but most conspicuously the borders, is covered by a 

 comparatively dense ciliated epithelium. Cilia do not exist in the 

 Crustacea. It is possible that these vibratile appendages may 

 exist on the branchise of Gasteropod Mollusks, and not on those 

 of Crustacea, because in the former the blood moving in the ulti- 

 mate vessels can be exposed to the agency of the aerating ele- 

 ment only on one side ; whereas, as formerly explained, in almost 



* I would here confess to the naturahst who may perchance repeat 

 these observations, that no researches in which I have ever engaged have 

 requu'ed so much training of the eye, ^xnk familiarizing of the mind v;'\.t\\ 

 the appearances under study, as the ultimate characters of the vascular 

 apparatus of the gills in the Cephalophorous Mollusca. Numerous diffi- 

 culties occur. The same doubtful point must be tested in very different 

 modes, in the recent and preserved specimens, and by aid of various che- 

 mical agents. In the Acephalous Mollusks, as in the Crustacea, the ulti- 

 mate blood-channels become vmquestionable at the first glance. Not so in 

 the branchiferous Gasteropods. The vessels are smaller and covered with a 

 denser epithelium ; the tissue is contractile and softer, the parts of difficult 

 access, &c. ; but notwithstanding these difficulties, I believe that the real 

 and true anatomy of these parts is faithfully given in the present memoir. 

 It is the first occasion in comparative anatomy on which an attempt has 

 been made to unravel the ultimate character of any part of the circulating 

 system of the Mollusca. I am disposed to attach importance to what is 

 true of the branchi;e in this sense, since it may hereafter prove of service 

 in deciphering the last vessels of other tissues and organs. I would only 

 in this place and at present venture to observe, that the lacunar theory of 

 Milne-Edwards is incontestably in every sense more applicable to the 

 Crustacea than to the Mollusca. 



