CONCHIFERS. 715 



Here the eggs remain for development. Since the young Anodonts 

 and Unios in the early period of life have a form differing from 

 the perfect animal, they have by some writers been regarded as 

 parasites that live in the gills 1 . It is almost needless to say that, 

 in the Lamellibranckiata, where the sexes are distinct, there is no 

 copulation. The eggs must leave the ovarium previously to im- 

 pregnation. 



The nervous system was by POLI, whose investigations have 

 contributed so much to the knowledge of the structure of con- 

 chifers, described as aqui-vascular system, since the wide neurilema 

 admitted impletion with quicksilver. MANGILI contested this 

 opinion. He observed four nervous ganglia in Anodonta, two at 

 the side of the mouth, one, the largest of all, in the foot, and a 

 fourth under the posterior occludent muscle of the shell, from which 

 ganglion the nerves for the gills and the mantle arise. All these 

 ganglia are mutually connected by nerve-filaments 2 . This may be 

 regarded as the first exposition, in some measure accurate, of the 

 nervous system in this class. As the rule, three central nerve- 

 masses may be admitted in the Lamettibranchiata, each consisting 

 of two lateral ganglia, of which, however, the first two alone are 

 constantly distinct from each other. These lie near the mouth, and 

 are connected by a longer or shorter transverse arched filament, a 

 commissure which runs in front of the mouth. On each side there 

 arises from these ganglia a long nerve-cord that runs to the poste- 

 rior nerve-mass, the ganglia abdominalia, often connected in a single 

 ganglion, and thus forms a wide ring which may be compared to 

 the cesophageal ring in other invertebrate animals. The second 



respiration, so generally attributed to them ; to say nothing of the male individuals in 

 which the temporary function for the gills of brooding organs entirely falls to the 

 ground. 



1 J. RATHKE (Shrivter of nat. SehTcabet, iv. i, 1797, pp. 166, 167, and after him 

 L. JACOBSON Kongel. Danske Vidcmlcab. Selskabs Naturvid. og Mathem. Afhandlinyer, 

 III. 1828, pp. 268, 269), described the animal living in the gills as a distinct species 

 under the name of Glochidium. The observations of V. BAEE (MECKEL'S Arckiv, 

 1830, s. 331) and especially of CARUS (Neue Untersuchungen ub. d. Entwickelungsgesch. 

 unserer filussmuscJiel, Leipsig, 1832, 4to, printed separately from the Act. Acad. Cces. 

 Leop. Carol, xvi. i) have now cleared this matter up. The young animals have 

 a triangular bivalve shell, which gapes open, and has at the outer margins an 



.appendage or hook turned inwards and furnished with small spines. Comp. also 

 QUATREFAGES, Ann. des Sc. Nat. sec. SeVie, v. 1836, Zool. pp. 321 336, PI. 12. 



2 Nuove Richerche zootomiscke sopra alcuni specie di Conchiglie bivalvi, Milan o, 

 1804 ; translated into German in BEIL'S Archiv, IX. s. 213 -220, Taf. x6. 



