3.20 Dr. T. Williams on the Mechanism of Aquatic 



The spacious sinuses and lacunae developed in the course of 

 the venous half of the circulation are not said by this author to 



comme je I'ai deja dit, par la masse charuue de la bouche, les glandes sali- 

 vaires, les principaux ganglions dii systeme nerveux, et un grand nombre 



de brides musculaires et fibreuses." " Mais un fait qui, an premier 



abord, paraitra plus singulier encore, c'est que, tandis qu'une portion de la 

 cavite geuerale vient completer I'appareil vasculaire, I'artei'e aorte remplit 

 des fonctions analogues a celles de la cavite abdominale, car elle loge dans 

 sou interieur vme portion de I'appareil digestif." It does not clearly ap- 

 pear fi-om the observations of M. Quatrefages, in his memoir " Sur la Cavite 

 gene'rale du Corps des Invertebres" (Ann. des Sci. li Ser. tom. xiv. 1850), that 

 he has specially considered this point. He does indeed in one place positively 

 state, that the " abdominal cavity in the Acephalan and Gasteropod Mol- 

 lusks is a part or segment of the circulatory system" : — " Chez les Mol- 

 lusques Acephales, proprement dit, I'existence de la cavite geuerale est 



encore plus completenient deguisee Les Mollusques Gastero- 



jjodes presentent souvent quelque chose de semblable. Toutefois, chez 

 ces derniers la cavite generale est presque toujours tres reconnaissable, 

 surtout dans la partie qui correspond au pied." Here no distinction is 

 attempted between the true peritoneal cavity, such as it exists in the 

 Radiated aad Annulose classes, and those lacunse or vascular dilatations 

 such as Milne-Edwards iirst described in the structure of the Mollusca. 

 This point is one of great zoological as well as homological interest. If in 

 the Mollusca the circulatory system is ])eripherically nothing but a ramifica- 

 tion of the peritoneal cavity, it is only a chylaqueous system plus a heart. 

 But if it can be proved that the peritoneal space is obliterated in these niol- 

 lusks, and that the venous lacunae of Milne- Edwards are parieted, contrac- 

 tile, dilated vessels, that is, segments of the fluid system, situated in the inter- 

 stices of the solid organs, every reason will have been overthrown for sup- 

 posing that the apparatus of the circulation in the mollusks is nothing but 

 a ramified development of the peritoneal cavity. In all animals below the 

 Mollusca, the system of the i)erivisceral chamber is dedicated to a distinct 

 and separate system of nutritive fluids. Its parietes in all cases are non- 

 contractile— almost abvays ciliated at some point or other of its extent — 

 while the fluid contents are invariably circulated by the action of externally 

 situated muscles. These are peculiarities which appear to isolate this 

 system almost completely from the circulatory apparatus of the Mollusca ; 

 in other words, they seem to prove that it partakes more fully of the cha- 

 racters which belong to the maturely developed circulation of the Verte- 

 brated animal, than of those of the rudimentary chylaqueous system of the 

 Radiate and Annulose classes. If the arterial half of the circulation of 

 Mollusks be likened to the blood-proper system of Annelids, and the 

 venous half of the former be taken as the representative of the perivisceral 

 cavity in the latter, the homology of the molluscan fluid-system would be 

 established ! Siebold observes^" In Nudil)ranchs, Cyclobranchs, Scuti- 

 branclis, 'J'ectibranchs, Pectinibranchs, and Pulmonata, &c., these venous 

 canals are only lacunte excavated in the muscular walls of the body, and 

 are without ])roper walls, as Meckel has pretended is the case with those of 

 yfplysia;" and Souleyet (Coniptcs Rendus, xx. p. 81, note 3) remarks, 

 "que le systeme veineux des Mollusques n'est pas toujours forme par des 

 vaisseaux distincts, mais qu'il se com])ose en grande partie de ces canaux 

 crcuses dans re])aisseur ou dans I'interstice des organes." And it has been 

 sup[)oscd by Prof. Owen and others, that the heart-like dilatations which 

 occur at various points in the circulation of the Cephalopods, arc lined 



