156 



MOLLUSCA 



mouth (fig. 144, B, a) and connected in front of it by a 

 commissure, are the representatives of the cerebral, pleural, 

 and visceral ganglia of the typical Mollusc, which are not 

 here differentiated as they are in Gastropods (compare, 

 however, fig. 67). A pair placed close together in the foot 

 (fig. 144, B, b, and fig. 

 124, (6), ax) are the typ- 

 ical pedal ganglia ; they 

 are joined to the cerebro- 

 pleuro - visceral ganglia 

 by connectives. 



Posteriorly beneath 

 the posterior adductors, 

 and covered only by a 

 thin layer of elongated 

 epidermal cells, are the 

 olfactory ganglia, their 

 epidermal clothing con- 

 stituting the pair of os- 

 phradia, which are thus 

 seen in Lamellibranchs 

 to occupy their typical 

 position and to have the 

 typical innervation, the 



TIPT-VP fn pa nil nsnlirarl Fla 144. Nerve-ganglia and cords of three 



OSpJiraa- La memt , ranchs (from Gegenbaur): A, of 



ium being given off by Teredo ; B, of Anodonta ; C, of Pecten. a, 



,1 i v cerebral ganglion-pair ( = cerebro-pleuro- 



tne Visceral ganglion visceral) ; 6, pedal ganglion-pair ; c, olfac- 



that is to say, by the tory (osphradial) ganglion-pair. 



undifferentiated cerebro-pleuro- visceral ganglion of its 

 proper side. This identification of the posterior ganglion- 

 pair of Lamellibranchs is due to Spengel (11). Other 



\ 





FIG. 145. Pallial eye of Spondylus (from Hickson). a, pree-corneal epithe- 

 lium ; b, cellular lens ; c, retinal body ; d, tapetum ; e, pigment ; /, retinal 

 nerve ; g, complementary nerve ; h, epithelial cells filled with pigment ; k, 

 tentacle 



anatomists have considered this ganglion-pair as corre- 

 sponding to either the pleural or the visceral of Gastropoda, 

 or to both, and very usually it is termed "the parieto- 

 splanchnic " (Huxley). 



The sense-organs of Anodon other than the osphradia 

 consist of a pair of otocysts attached to the pedal ganglia 

 (fig. 124, (6), ay). The otocysts of Cyclas are peculiarly 

 favourable for study on account of the transparency of the 

 small foot in which they lie, and may be taken as typical 

 of those of Lamellibranchs generally. The structure of 

 one is exhibited in fig. 146. A single otolith is present 

 as in the veliger embryos of Opisthobranchia. In adult 

 Gastropoda there are frequently a large number of rod-like 

 otoliths instead of one. 



Anodon has no eyes of any sort, and the tentacles on the 

 mantle edge are limited to its posterior border. This 

 deficiency is very usual in the class; at the same time, many 

 Lamellibranchs have tentacles on the edge of the mantle 

 supplied by a pair of large well-developed nerves, which 

 are given off from the cerebro-pleuro-visceral ganglion-pair, 



and very frequently some of these tentacles have undergone 

 a special metamorphosis converting them into highly- 

 organized eyes. Such eyes on the mantle-edge are found 

 in Pecten, Spondylus, Lima, Ostrea (?), Pinna, Pectunculus, 

 Modiola, Mytilus (?), Cardium, Tellina, 

 Mactra, Venus, Solen, Pholas, and Ga- 

 leomma. They are totally distinct from 

 the cephalic eyes of typical Mollusca, and 

 have a different structure and historical de- 

 velopment. They have not originated as 

 pits but as tentacles. They agree with the Fm 146 _ oto t 

 dorsal eyes of Onchidium (Pulmonata) in of Cyclas (from 

 the curious fact that the optic nerve pene- ra^uie^fciiiated 

 trates the capsule of the eye and passes in ceils lining the 

 front of the retinal body (fig. 145), so that 

 its fibres join the anterior faces of the nerve-end cells as 

 in Vertebrates, instead of their posterior faces as in the 

 cephalic eyes of Mollusca and Arthropoda ; moreover, the 

 lens is not a cuticular product but a cellular structure, 

 which, again, is a feature of agreement with the Vertebrate 

 eye. It must, however, be distinctly borne in mind that 

 there is a fundamental difference between the eye of Verte- 

 brates and of all other groups in the fact that in the 

 Vertebrata the retinal body is itself a part of the central 

 nervous system, and not a separate modification of the 

 epidermis myelonic as opposed to epidermic. The struc- 

 ture of the reputed eyes of several of the above-named 

 genera has not been carefully examined. In Pecten and 

 Spondylus, however, they have been fully studied (see fig. 

 145, and explanation). 



The gonads of Anodon are placed in distinct male and 

 female individuals. In some Lamellibranchs for instance, 

 the European Oyster and the Pisidium pusillum the sexes 

 are united in the same individual; but here, as in most 

 hermaphrodite animals, the two sexual elements are not 

 ripe in the same individual at the same moment. It has 

 been conclusively shown that the Ostrea edulis does not 

 fertilize itself. The American Oyster (0. virginiana) and 

 the Portuguese Oyster (0. angulata) have the sexes sepa- 

 rate, and fertilization is effected in the open water after 

 the discharge of the ova and the spermatozoa from the 

 females and males respectively. In the Ostrea edulis fertil- 

 ization of the eggs is effected at the moment of their escape 

 from the uro-genital groove, or even before, by means of 

 spermatozoa drawn into the sub-pallial chamber by the in- 

 current ciliary stream, and the embryos pass through the 

 early stages of development whilst entangled between the 

 gill-lamellae of the female parent (fig. 6). In Anodon the 

 eggs pass into the space between the two lamella? of the 

 outer gill-plate, and are there fertilized, and advance whilst 

 A ,.. B 



p-ad 



at 



FIG. 147. Two stages in the development of Anodonta (from Balfour). Both 

 figures represent the glocliidium stage. A, when free swimming, shows the 

 two dentigerous valves widely open. B, a later stage, after fixture to the fin 

 of a fish, sh, shell ; ad, adductor muscle ; s, teeth of the shell ; by, byssus ; 

 a. ad, anterior adductor ; p.ad, posterior adductor ; mt, mantle-flap ; /, foot ; 

 br, branchial filaments ; au.v, otocyst ; a!, alimentary canal. 



still in this position to the glochidium phase of develop- 

 ment (fig. 147). They may be found here in thousands 

 in the summer and autumn months. The gonads them- 

 selves are extremely simple arborescent glands which open 

 to the exterior by two simple ducts, one right and one 



