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!. It is almost needless to say that, 
in the Lamellibranchiata, where the sexes are distinct, there is no 
copulation. The eggs must leave the ovarium previously to im- 
pregnation. 
The nervous system was by Pout, whose investigations have 
contributed so much to the knowledge of the structure of con- 
chifers, described as aqui-vascular system, since the wide newrilema 
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*, 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 Lamellibranchiata, 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. Selskabet, tv. 1, 1797, pp. 166, 167, and after him 
L. Jacosson Kongel. Danske Videnskab. Selskabs Naturvid. og Mathem. Afhandlinger, 
ur. 1828, pp. 268, 269), described the animal living in the gills as a distinct species 
under the name of Glochidium. The observations of V. Barr (MECKEL’s Archiv, 
1830, s. 331) and especially of Carus (Neue Untersuchungen iib. d. Entwickelungsgesch. 
unserer Flussmuschel, Leipsig, 1832, 4t0, printed separately from the Act. Acad. Cos. 
Leop. Carol. Xvi. 1) 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. Série, v. 1836, Zool. pp. 321—336, Pl. 12. 
2 Nuove Richerche zootomische sopra alcuni specie di Conchiglie bivalvi, Milano, 
1804 ; translated into German in REtIu’s Archiv, IX. s. 213220, Taf. xd. 
