Respiration in Invertebrate Animals. ' 819 



since these mollusks are seldom out of the water, this mode of 

 action can scarcely at any time be put in request. They fi^ 

 themselves by the action of the foot alone. By the undulat'^g 

 motion of this organ, the water between it and the surface of 

 the stone is pressed out. The soft parts within the shell are 

 then subject to the uncounterbalanced pressure of the superin- 

 cumbent column of water which operates through the orifice at 

 the apex of the shell. In the case of the Patella, it is by the 

 shell that the atmospheric pressure is borne. The vacuum is 

 formed, not by the extrusion of the water from the roof of the 

 shell, but by the adaptation of the foot to the surface of contact. 

 Hydrostatic or atmospheric pressure, as the case may be, becomes 

 thus a considerable assistant force, but it does not, as commonly 

 supposed, constitute the only and exclusive mechanism by which 

 these mollusks cling to the rock. 



These observations therefore justify the total and uncon- 

 ditional exclusion of the abdominal region of the body in the 

 Conchiferous Univalves from the office of aerating the fluids. 

 Neither the investing membrane nor the distribution of the 

 blood underneath sanctions this idea. 



This conclusion is not opposed to the views first stated by 

 Milne-Edwards, in his celebrated essay " Sur la Circulation chez 

 les MoUusques *." 



♦ Annales des Sciences, 3 Sex. torn. viii. 1847. Nowhere does this 

 distinguished observer describe au abdominal cavity in the MoUusca. 

 Nothing in this class in the adult state exists which can be compared to the 

 free, undivided visceral cavity of the Echinoderms and Annelids. Here 

 this space is occupied by an independent fluid, the chylaqueous. In Mol- 

 lusca such a fluid does not exist. The cavity therefore is not required. 

 Milne-Edwards chiefly insists upon the fact, that the venous system is de- 

 ficient or imperfectly developed in this class. " Dans tons les Mollusques 

 dont la structure nous est connue, les vaisseaux sanguins manquent en 

 partie, et line portion plus ou moins considerable du cercle circulatoire se 

 trouve constituee par de simples lacunes." In another place he observes — 

 " Mais dans la tete, je voyais toujours I'injection s'extravaser et remplir 

 une grande cavite oil se trouvent loges le cerveau, les glandes salivaires, le 

 pharynx, et tons les muscles de la bouche." Again, he speaks in Haliotis 

 of " une communication libre et normale entre la grande artere du corps et 

 la cavite cephalique ou se trouvent loges les principaux centres nerveux et 

 toute la portion anterieure de I'appareil digestif." He then states that this 

 " cavite cephalique " is filled with arterial blood. In a still more definite 

 manner he thus describes the only " cavity " for the reception of fluid 

 which exists in the Molluscan organism : — " Effectivement, je me suis 

 assure que, chez ce grand Mollusque Gasteropode, I'artere aorte, parvenue 

 au i)oint ou le canal digestif se recourbe pour descendre de la face supe- 

 rieure du bulbe pharyngien dans la cavite abdominale, debouche directement 

 dans une vaste lacune, dont les parois sont formees en partie par les tegu- 

 ments communs de la tete, et en partie par les muscles et les tuniques du 

 pharynx jointes a des lames de tissu connectif etendues transversalement 

 au (levant de la cavite abdominale, lacune dont I'interieur est occupe. 



