20 INTRODUCTION. 



the ramifications of the ahmentary canal are also described as penetrating the branchiae, 

 "where the chyle was directly submitted to the atmospheric influence exercised by the 

 surrounding water." To this supposed condition of the digestive, circulating, and respi- 

 ratory systems, he gave the name, ' Phlebenterism,' and proposed thereon some corres- 

 ponding changes in the classification of the mollusca. 



In the 'Report' on this and other memoirs of M. Quatrefages on the Nudi- 

 branchiate Mollusks, by M. Milne Edwards, that distinguished professor adopts the 

 mode of interpreting the modification of the venous system, and applies it to the 

 Crustacea. " II existe" (dans le genre Eolidine) un coeur et des arteres bien constitues, 

 mais pas des veines proprement dites, et le sang ne revient des divers parties du corps 

 que par un systeme des lacunes irregulieres, disposition tout-a-fait analogue a celle dont 

 les Crustacees nous avaient deja fourni un example. Enfin dans d'autres especes, que 

 M. Quatrefages a decouverte sur les cotes de la Bretagne, le coeur et les arteres 

 disparaissent a leur tour; de sorte que la circulation devient des plus incompletes et 

 resemblent a celle qu'on appergoit chez les Bryoozoaires." Subsequent researches by 

 Messrs. Embledon and Hancock demonstrated, however, the existence in the genera 

 Chalide and Acteonie, of both heart and arteries ; and the same careful anatomists confirm 

 the observations of M. Souleyet,^ that the veins were not wanting in the nudibranchs, but 

 had only undergone that modification of form to which the term sinuses is more 

 properly given. Not any of these authors have, however, published exact and recog- 

 nisable representations of the relations of the attenuated tunica propria of the veins, to the 

 interspaces or laminae which that tunic lines in forming the large and irregular sinuses in 

 which the blood is diff'used. With a view of supplying this desideratum, further illustra- 

 tions of the circulating system of the Brachiopoda are added here to those which were 

 published in my earlier memoirs.^ Figure 1, pi, 3, more especially will show that the venous 

 system of the Terebratula, like that of other mollusks, is continuous, i. e. forms a part by 

 continuity of tissue, with the rest of the circulating system. Thus all ulterior researches 

 rightly interpreted, since the time of Hunter, have served to confirm and establish the 

 accuracy of his appreciation and explanation of the facts he discovered in insects and 

 crustaceans, and to extend our knowledge of the same modification of the venous 

 system as it exists in the general series of mollusks. 



The absorbent system, as Cuvier truly states,^ is wanting in mollusks. In the 

 comparatively low organised form under consideration {Terebratula), the chyle or product 

 of digestion must transude through the intestinal tunics into the intestinal sinus, pi. 3, 

 fig. 1,8: all the fluid answering to lymph, fi'om other parts of the organization will be 



^ Observations sur les Mollusques Gasteropodes design^s sous le nom de ' Phlebentferes ' par M. de 

 Quatrefages ; Coraptes Rendus des Sciences, &c., 1844, t. xix, p. 355. 



2 Trans. Zool. Soc, 1833, pi. xxii, figs. 7, 8, 11 ; pi. xxiii, figs. 11, 16. — Annales des Sciences Nat., 

 pi. iv, figs. 8, 9, and 10. 



3 Tom. cit., p. 300. 



