24 A MONOGRAPH OF THE 



powers as the Nudibranchs should be ordained to make perilous migrations, while the well 

 protected testaceous gasteropods are permitted to enjoy a sedentary existence. The latter 

 might roam about in comparative security; and, indeed, trusting to their shelly covering, they 

 do boldly expose themselves on rocks and sea-weeds. The Nudibranchs, on the contrary, 

 avoid the light, their nakedness compelling them to seek concealment in crevices of rocks and 

 under stones, where they probably pass the whole of their lives ; and being thus, as it were, 

 confined to a spot, their dissemination is provided for by the natatory condition of the larvae ; 

 without which provision these animals would probably be extremely local. The way in which 

 the species are distributed between tide-marks is also unfavorable to the notion of migration . 

 as most of them inhabit well defined zones or belts on the beach, beyond which they are rarely 

 found. 



The periodical appearance of the Nudibranchs within tidal range admits of another and 

 more satisfactory explanation; namely, that these animals, whose period of life is probably 

 short, may not live longer than a year, and that most of the old individuals die off after the 

 breeding season, and the young fry, being for many months afterwards small and inconspicuous, 

 escape observation, attaining maturity only a short time before the next breeding season, 

 when, having given birth to a new progeny, they die and disappear in their turn. 

 Whatever be the cause, the various species generally do disappear from the rocks shortly 

 after their respective breeding seasons, though for some months afterwards solitary individuals 

 occur. In the course of two or three months the fry may be detected if closely looked for, 

 and afterwards their development noted until they reach maturity and shed their spawn in the 

 same localities that were occupied by their parents. We have in this way traced the 

 development of Goniodoris nodosa throughout the year, and can vouch for its attaining its full 

 growth in twelve months, as, indeed, is evident enough in most of the common rock species. 

 It is true that the disappearance of the adult individuals after spawning may be accounted for 

 by their retiring into deeper water; but, in this case, we might expect to be able to trace their 

 change of place on the shore at different seasons ; besides that, several of the most abundant 

 littoral species are scarcely, if ever, brought up by the dredge, and certainly never in such 

 numbers as to warrant the conclusion of their living for the greater part of the year below low- 

 water mark. Some of the species are to be found within tide-marks during the whole year, 

 though most plentifully in the breeding season. Such species most probably live longer than 

 one year. Other species, again, are taken within tidal range for a year or two, and then 

 disappear from the locality for a long period. Some of these species appear to belong to deep 

 water, and their accidental occurrence on the rocks may arise from the agency of currents 

 sweeping the natatory larvae beyond the limits of their proper zone. 



Not much is known concerning the longevity of the Nudibranchs, but if the opinion we 

 entertain respecting the periodical appearance of these Mollusks on the rocks be correct, it is 

 evident that the period of existence of numerous species must be short, — not much exceeding 

 a year. And, judging from the rapidity of their growth, and their great fecundity, compared 

 with the few individuals that are found in a mature state, it may be inferred that such is correct. 

 In some of the species, however, individuals of overgrown size and venerable appearance may 

 be met with that might be supposed to have lived through many summers. M. Bouchard 

 Chantereaux states that Eolis Cuvien {papillosd) does not acquire its full growth till the end of 



