PTEROPODA. 



171 



vicinity of the polar circle ; and, being emi- 

 nently constructed for a pelagic life, never 

 approach the shore. They are all, moreover, 

 nocturnal or crepuscular, voyagers agreeing 

 that they are never to be seen during a clear 

 day when the sun shines brightly ; but 

 towards five o'clock in the evening, when 

 the weather is cloudy, two or three species 

 begin to make their appearance at the surface 

 of the water, generally belonging to the genus 

 Hyalea. 



As soon as twilight begins, large quantities 

 of small Cleodorcc, Hi/alece, and Atlanta; may 

 be caught ; but the larger species only come 

 to the top when night has set in ; at which 

 time only the Pneumodermas, the C/ios, and 

 the large Clcodorts can be procured. Certain 

 species indeed only approach the surface on 

 very dark nights, as, for example, the Hyalea 

 balantium. Very soon all the smaller species 

 again gradually disappear, as do the large ones 

 a little later, and towards midnight a few 

 stragglers only of different kinds are to be met 

 with. At sunrise not a single Pteropod is to 

 be seen, either at the surface, or at any depth 

 to which the eye can penetrate. Each spe- 

 cies, in fact, seems to have its appropriate 

 hours, or rather its appropriate degrees, of 

 darkness. 



M. d'Orbigny supposes, from these habits, 

 that each species lives at a certain depth in 

 the water which is proper to it, and where it 

 is consequently exposed to a diminution of 

 light proportionate to its distance from the 

 surface. Every species, therefore, will only 

 come to the top at that period of the twenty- 

 four hours when the obscurity approximates 

 to that to which it is accustomed in its usual 

 situation while the sun is above the horizon, 

 mounting gradually upwards as the light of 

 day diminishes. If the Pteropoda remained 

 all night at the surface of the sea, there might 

 be reason to think, as M. Rang supposed, that 

 they ascend at sunset for the purpose of ob- 

 taining food or fresh air in the most super- 

 ficial strata of the ocean ; but as these could 

 be procured at all hours, it seems more pro- 

 bable that it is the light which thus regulates 

 their movements. 



There is reason to suppose that each spe- 

 cies of Pteropod remains during the whole 

 year in the same regions of the ocean. These 

 regions are of different degrees of extent, 

 and currents doubtless tend to enlarge their 

 boundaries ; probably to this cause must be 

 attributed the extensive diffusion of certain 

 species met with in all climates ; whilst others 

 of larger size are only found in the torrid 

 zone, and others again of equal dimensions 

 are peculiar to cold climates. 



A table appended to the Memoir of M. 

 d'Orbigny assigns the limits between which 

 each species has been found, and its nocturnal 

 or crepuscular habits. From this table it 

 appears, that of twenty-nine species of Ptero- 

 pods known to the author, fourteen are met 

 with both in the Atlantic and Pacific oceans, 

 whilst eleven are proper to the Atlantic ami 

 four to the Pacific ; of these seventeen are 



altogether nocturnal in their habits, and only 

 eleven crepuscular. 



The Pteropoda swim in a very peculiar 

 manner. Their cephalic fins are only able to 

 support them by a constant repetition of 

 rapid movements, resembling those of the 

 wings of a butterfly. These fins are kept in 

 motion continually ; and, according to the di- 

 rection of their stroke, the animal advances 

 horizontally, or mounts or descends, the body 

 remaining all the time either in a vertical 

 position or slightly inclined. Sometimes they 

 keep spinning round without changing their 

 place, or even keep at a certain height in the 

 water without any apparent exertion ; but 

 this power of remaining motionless has only 

 been observed in a small number of species, 

 the butterfly-movement of the wings being 

 most commonly resorted to. If while they 

 are thus in motion, the appearance of any 

 strange body or even a sudden shock given 

 to the vessel in which they are contained, 

 causes them alarm ; their wings fold upon 

 their bodies, or in some species are entirely 

 withdrawn into their shell, and the animal 

 sinks rapidly to the bottom of the vessel. 

 Most probably, when at liberty, as soon as 

 the creature has sunk to a sufficient depth to 

 ensure safety, it again unfolds its wings, and 

 sustains itself in the water instead of allowing 

 itself to go quite to the bottom. 



The Hyalea and Cleodora swim with the 

 greatest rapidity, in Pneumoderma and Clio 

 the movements are less vivacious. 



The larger Pteropods seem to feed prin- 

 cipally upon smaller species of their own class, 

 as well as upon the minute crustaceans that 

 swarm in the seas they frequent. 



CLIO. Integument. The skin of the Clio 

 is not smooth, but studded with numerous 

 little wart-like eminences, causing a roughness, 

 which is in direct relation with the red colour 

 of the integument, and is consequently most 

 conspicuous near the extremity of the tail. 

 Both the roughness and the red colour in- 

 deed are produced by the presence of a mul- 

 titude of little cavities or sacculi filled with an 

 oily red pigment, the pointed ducts of which 

 project externally. These pigment-sacs are 

 not only most abundant near the extremity 

 of the tail, but in that part of the body are 

 of larger size than elsewhere : they are all 

 flask-shaped, opening upon the surface of the 

 body by a narrow neck, while their larger ex- 

 tremity is imbedded in the subcutaneous cel- 

 lular tissue. Beneath these larger pigment- 

 sacs smaller ones of a similar description are 

 perceptible, much smaller in their dimensions 

 than the preceding, anil in many places where 

 the larger ones are deficient, the smaller pig- 

 ment cells are proportionately more nume- 

 rous : both kinds are filled with the same oil- 

 like colouring material, and are apparently 

 comparable to simple mucous follicles, only 

 their secretion is of a more oily character. 



With the exception of the pigment cells, the 

 integument of the living Clio is quite trans- 

 parent, but after being kept in spirits of wine, its 

 transparency is considerably diminished ; in its 



