MONOiCIOUS MOLLUSCA. 347 



of the contact of any vivifying fluid ; or, in other words, 

 that they are fecundated before they leave the ovaries, by 

 testes which must be conjoined with those organs. No dis- 

 tinct male organs appear to be present. Perhaps Home, 

 who mentions their existence, has mistaken the excretory 

 organs for them, as have many other anatomists. From what 

 the author has observed in the Modiolae and Mytili, he 

 believes that the organs called ovaria do, at certain periods, 

 secrete the seminal fluid, which impregnates the ova con- 

 tained in them, and is then discharged as an excretion by 

 the oviducts." * 



The Conchifera of European seas are big with spawn 

 usually in the spring, or at the commencement of summer, 

 and some species are said to be fruitful more than once in 

 the year. Previous to their expulsion, the young may be 

 seen within the pellicle of jelly by which they are sur- 

 rounded and enclosed, opening and closing their tiny valves, 

 and moving on their axis in a rotatory manner ; — " perhaps, 

 when most lively, there are seven or eight volutions in a 

 minute." f "When ripe for an independent existence the 

 loaded spawn is discharged from the oviducts, and carried 

 from the body of the parent, partly by means of the motions 

 of the closing valves, "but principally by the force of the 

 currents of the respired water. The young, speedily 

 loosened by the dissolution of the jelly in which they were 

 nurtured, are now disseminated through the water by ciliary 

 movements, and by their own locomotive powers ; for at 

 this, the very spring-day of their existence, even the very 

 oyster joys in its motility, the mussels roam at freedom, the 

 stone-boring Pholas and Lithodomus seek out rocks un- 

 visited by their progenitors, and the other burrowing tribes 



* Charlesworth's Mag. Nat. Hist. iii. 439. 



t This beautiful phenomenon was first observed by Leeuwenhoek; and 

 well described by him in the pond mussel. " Some of these mussels I 

 opened in the presence of the engraver, in order that, as soon as I had taken 

 some of the young ones out of their receptacles, he might make a drawing 

 of them, for were they suffered to stand but a few hours, their true figure 

 would be lost. The unborn mussels being put into a glass tube and placed 

 before the microscope, I saw with astonishment a most pleasing spectacle, 

 for every one of them, each in its particular membrane or covering, had a 

 slow circumvolution, and that not for a short space of time, but sucli turn- 

 ing round or rotatory motion was observable for three hours afterwards, and 

 it was the more curious, because the young mussels, during the whole of 

 their motion, constantly kept in the centre of their membranes, just as if 

 one were to see a sphere or globe revolving upon its axis. This uncom- 

 monly pleasing spectacle was enjoyed by myself, my daughter, and the 

 engraver, for three whole hours, and we thought it one of the most delight- 

 ful that could be exhibited." — Select Works, i. 87. 



