ACEPHALA LAMELLIBEANCHIATA. 23 



of this interesting and almost undeveloped class, to Professor 

 Owen's Memoir, vol. i. of the ' Zoological Transactions/ and 

 to M. Deshayes and Milne-Edwards's comment and observa- 

 tions, vol. vii. p. 305-329 of their second edition of Lamarck's 

 1 Animaux sans Vertebres/ 



We conclude by repeating our opinion that the Pallio- 

 branchiata are not of co-ordinate organization with the Lamel- 

 libraiichiata, but an inferior group, having many connective 

 links with the Cirripoda. Our notes are unavoidably scanty, 

 from the dearth of materials and difficulty of the subject. 

 We have done : as facts cannot be stated, hypothesis ought 

 not to supply their place. 



ACEPHALA LAMELLIBRANCHIATA. 



I shall now give an outline of the anatomy of the lamelli- 

 branchiate bivalves, to serve as a standard of comparison. 

 The internal structure of the animals of this division is less 

 complicated than in the Gasteropoda, and presents such a 

 similarity throughout the whole class, as to render it unne- 

 cessary to give more than a detailed account of the organs and 

 functions of the animals, without reference to a particular 

 species. This generalization will include all the genera of the 

 subsequent groups of the Acephala that may be noticed, 

 except that especial care will be had to note any essential 

 departure from the general organization in any particular 

 genus or species. 



The circulation, including the respiration, is complete ; it 

 usually consists of a pair, of infinite variety of outline, of 

 suboval vascular plates, fixed under the mantle on each side 

 the body ; these are composed of a network of branchial tubes, 

 and are each furnished with an arterial and branchial vein, 

 the one to receive the blood from the vense cavse for aeration, 

 and the other to convey it through the auricles to the heart, 

 to be again transmitted into the system ; the artery of each 

 lamina is situate at the upper margins of the branchiae, at 



