THE LAMELLIBRANCHIATA. 413 



line. When the siphons are largely developed they have re- 

 tractor muscles, the insertions of which are so disposed as to 

 cause the posterior part of the pallial line to be more or less 

 deeply curved or angulated. Hence the distinction of integro- 

 palliate and siniipalliate as applied to Lamellibranchs which 

 have the pallial line evenly rounded or notched. 



The cerebral ganglia lie at the sides of the mouth, and 

 are connected by a commissure, which passes in front of it. 

 They give branches to the anterior region of the mantle, to 

 the gills, to the anterior adductor muscle, to the labial palpi, 

 and to the parts about the mouth. The pedal ganglia are situ- 

 ated in the foot; or in the corresponding region on the neu- 

 ral side of the alimentary canal, when no foot is developed. 

 Each is united by a commissure with the cerebral ganglion of 

 the same side, and gives off branches to the muscles of the 

 foot. The parieto-splanchnic ganglia lie on the neural face 

 of the posterior adductor muscle. The long commissures which 

 unite them with the cerebral ganglia usually traverse the 

 renal organ, and lie beneath the floor of the pericardium. 

 Each of these ganglia gives off a nerve to the branchia of its 

 side, and supplies the posterior and middle part of the man- 

 tle. This posterior pallial nerve may anastomose with the 

 anterior pallial nerve from the cerebral ganglion. The gan- 

 glia also furnish nerves to the posterior adductor muscle, to 

 the heart, to the rectum, and to the muscles of the siphons, 

 when the latter are present. Eyes are never developed in the 

 cephalic region of the Lamellibranchs, but, in many (e. g., 

 Pecten), numerous simple eyes terminate papillae of the mar- 

 gins of the mantle. Auditory sacs are almost invariably at- 

 tached by longer or shorter peduncles to the pedal ganglia. 



The Lamellibranchiata are usually dioecious, but some- 

 times hermaphrodite ' (e. g., Ci/clas, some species of Cardium 

 and Pecten, Ostraea, Clavarella, and Pandor a). The genera- 

 tive organs are ramified glands of simple structure and simi- 

 lar in both sexes, the ducts of which open into, or close to, 

 the renal organs. 



The process of yelk-division s usually gives rise to smaller 



1 The testes and ovaria are distinct in the hermaphrodite Perth/ ex. In Cat" 

 dium serratum, adjacent c;eca of the sexual gland contain spermatozoa or ova, 

 or both products may be developed in the same caecum. In the common Oys j 

 ter the genital cax'a in any given individual are found to be either almost all 

 ovigerous or almost all spermigerous ; and it appears probable that the pre- 

 dominantly male precedes the predominantly female condition. See Lacaze- 

 Duthiers, " Organes genitaux des Acephales Laniellibranches." (" Annates 

 des Sciences Naturelles" 1854.) 



2 See Loven, ArcMvfur NaturgeschicMe, 1849. De Quatrefages, " M^moires 

 sur l'Embryogenie des'Tarets." (" Annales des Sciences Naturelles," 1849.) 



