118 Dr. 0. A. L. Morch on the Operculum and its Mantle. 



the line of suture. The shell of the young Dentalium is also 

 split throughout its whole length. The best proof of analogy 

 would perhaps be the carapace of Limnadia and Est heria, among 

 Crustacea, which is bivalve, while that of closely allied genera, 

 as Nehalia and Apus, is univalve. 



Pinna saccata, L., has both valves united in the adult state ; 

 but it has never been observed that the two valves have their 

 origin in the division of a single shell: on the contrary, the 

 division is manifest in the larval shell. 



Nearly all organs are double in the Acephala : there are thus 

 two ovaria with distinct external orifices, two kidneys (organs 

 of Bojanus) with distinct external orifices, two pairs of labial 

 palpi, two pairs of gills. It seems to me therefore probable that 

 the Acephala also have two shells originating in the same way 

 as the other organs above mentioned. This duplicity of the or- 

 gans is very indistinct among the univalves, as in Dentalium and 

 Chiton, and it becomes rarer and rarer among the higher 

 Mollusca. 



The larva {Glochidium) of Anodonta has in each shell a distinct 

 byssus-bundle ("cordons ombilicaux," Quatrefages*), and a dis- 

 tinct intestinal channel with distinct oral orifices f; in other 

 words, it is a true Diplozoon in the larval state. This curious 

 fact is perhaps not quite solitary among Mollusca. Thus, ac- 

 cording to Koren and Danielssen J, several eggs (from 1 to 100) 

 are, in Buccinum undatum, united to form a single embryo. The 

 diflference is chiefly that in the much lower mollusk, Anodonta, 

 the amalgamation takes place in a more advanced state of the 

 embryos, so that some organs, the intestinal channel and the 

 byssus, are united into one, and the other organs are kept in 

 their original condition. The animal would then be composed of 

 two "zonites," reminding us of the "egg-producing process'' 

 of Hydra, regarded by Prof. Huxley as a reduced individual, 

 or an organ homologous with an individual §. An Acephalous 

 mollusk must therefore be considered an individual in the same 

 sense as a plant or flower composed of individuals (leaves) reduced 

 to organs. The question is. Does the opercular lobe with its 

 operculum represent one lobe of the mantle and its shell in the 

 bivalves, or is it something difl^erent ? 



The epipodial line of Huxley (" manteau inferieur,'' Lacaze- 



* Ann. des Sc. Nat. torn. iv. (1835) p. 283, and torn. v. (1836) p. 321, 

 &c. 



t Von Siebold, Vergleichende Anatomie, Wirbellose Thiere, p. 294. 



X Bidrag til Pectinibranchiernes Udviklingsbistorie ; " On the Develop- 

 ment oi Buccinum undatum" (Athenaeum, 1852, p. 1066). 



§ Lecture upon Animal Individuality, Royal Institution (Ann. Nat. 

 Hist. ser. 2. vol. ix. p. 505). 



