Invertebrate Animals of the Western Coast of France. 29 



but are widely separated from the normal type of that group by 

 the structural conditions under which the functions of circulation, 

 respiration and digestion are performed. The great physiological 

 distinction in the nature of the circulatory apparatus of the class 

 Mollusca and Articulata consists in its being provided in the 

 former with two systems of membranous vessels united at one 

 end by the intervention of a heart, and communicating at the 

 other by a network of capillaries, while in the latter one of these 

 systems (the afferent or venous) is always wanting, and is sup- 

 plied by lacunae or intervals between the different organs, within 

 which the blood flows. Some years ago M. Quatrefages had de- 

 termined the fact, that in the compound Ascidia and several other 

 molluscoid animals, the vascular system only existed in the tho- 

 racic region of the body, and was replaced throughout the abdo- 

 men by interstitial meatuses resembling those in the Articulata ; 

 and that in the Bryozoa the inferior representatives of the same 

 zoological type, there existed no blood-vessels whatever, and the 

 nutrient fluid was distributed through large cavities of the body. 

 Hitherto however no true mollusk was known in which the cir- 

 culation was not completely vascular, nor could it have been well 

 anticipated that one of the highest groups of the class should 

 present the contrary character ; still the Eolidians and other 

 analogous Gasteropoda have furnished such a structural degra- 

 dation in different degrees. In the first a well-developed heart 

 and arteries exist, but no proper veins, the blood being returned 

 by means of a system of irregular lacunae similar to those met 

 with in the Crustacea; while in other species both the heart and 

 arteries have disappeared, and the circulation becomes as incom- 

 plete as in the Bryozoa. 



Corresponding modifications are entailed by the above in the 

 structure of the respiratory organs. There are no branchiae or 

 pulmonary sacs in the present Gasteropoda, as in the ordinary 

 Mollusca : respiration is either simply exercised by the general 

 surface of the integument, or limited to particular appendages 

 upon the back of the animal ; but even in the latter case no vas- 

 cular network enters into their composition, and to supply this 

 deficiency, nature has introduced a combination of the digestive 

 with the respiratory system, that was hitherto believed to occur 

 only in the Medusae and different Entozoa. The digestive cavity 

 gives off a system of canals, the ramifications of which penetrate 

 the branchiform dorsal appendages, and within these the nutri- 

 tive matters, being directly conveyed, are submitted to the influ- 

 ence of the air before being sent to the various parts of the body. 

 This complex vasculo-gastric system has been elaborately studied 

 by M. Quatrefages in the genus Eolidina ; in others it is con- 

 structed upon a more simple plan, reminding us of that of the 



