BRITISH NUDIBRANCHIATE MOLLUSCA. 25 



the second year ; and M. Nordmann thinks that his Tergipes Edwardsii lives at least two years. 

 The period will, no doubt, vary in different genera and species, though in none is it perhaps 

 much extended. 



The Nudibranchs, notwithstanding that they are androgynous, frequently copulate during 

 the breeding season. The conjoined individuals lie side by side, their heads turned in opposite 

 directions. Thus the right sides of the two animals are brought into close contact, and mutual 

 impregnation is effected. They remain in this position for some time, and in a short period 

 after separating, generally about the first or second day, the spawn is deposited. This is 

 usually adherent to some foreign body, and on expulsion from the animal is enveloped in a 

 perfectly transparent mucus, which at first is tenacious, having the property of adhering to 

 whatever it comes in contact with ; but the surrounding water soon destroys this quality, and, 

 to some extent, hardens it. By means of this mucus, the spawn becomes at once glued, as it 

 were, to the surface of whatever substance the animal may happen to be resting upon. While 

 shedding the spawn it slowly and gradually moves backwards in a spiral direction, beginning 

 in the centre ; thus the spawn assumes the spiral form, coiling in the usual way from right to 

 left. Occasionally, it is found coiled in a contrary direction, in which case the animal must 

 crawl forward during the propulsion of the spawn.* The spiral form is the mostperfect when 

 the animal is resting upon a flat surface : on corallines, for want of an expanded support, it 

 necessarily becomes more irregular. The form and caliber of the sexual aperture and the even 

 or interrupted propulsion of the spawn, combine in different degrees with internal organic 

 causes, to form those peculiar and elegant patterns which characterise the different species. 

 When the spawn is shed freely and at once, the masses assume an oval or kidney shape ; but 

 a more slow and gradual propulsion results in its taking a spiral form from the causes already 

 named. 



The time required for the maturity of the embryo after the deposition of the spawn varies 

 in different species, and under different circumstances, from a few days to a month or more ; 

 the usual time appears to be about ten days or a fortnight. The extraordinary difference between 

 the minute natatory larva in its first state of existence and the adult animal, has been fully 

 described in another part of this work.f That a minute creature, scarcely visible to the naked 

 eye, enclosed in an operculated shell, and swimming freely through the water by means of 

 ciliated lobes, should turn into the large and sluggish Doris tuberculata or Tritonia Hombergii, 

 is one of nature's romances, only to be learnt from a careful study of her works. Unfortunately, 

 though their development from the spawn can readily be traced, naturalists have not hitherto 

 succeeded in preserving these delicate little creatures alive for more than a few days, so that 

 the progress of their change into the perfectly formed animal has not been traced in detail. 

 Thousands, no doubt, perish at an early stage, or become the prey of other marine animals. The 

 accidents to which they are liable, or the occurrence of circumstances favorable to their pre- 

 servation and diffusion, may readily account for the occasional appearance and disappearance 

 of some species in a manner apparently, at first sight, somewhat arbitrary. 



* The figures of the spawn in different works, in consequence of the inattention of the engravers, 

 are often reversed, and cannot be relied upon. 



f See the genus Doris, anatomy and development. 



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