122 GEOGRAPHICAL DISTRIBUTION. 



as in Argonauta, Cleodora, Atlanta, Carinaria, lanthina, &c. Ab- 

 normalities of structure, especially in the case of the immature forms 

 of littoral species, are frequent, leading to such modification of out- 

 line as to obscure in great measure the general parental relationship. 

 The supposed young of the conger-eel develop into small transpar- 

 ent ribbon-shaped fishes, largely devoid of haemoglobin in their 

 blood, and with an exclusively cartilaginous skeleton ; the young 

 of certain flat-fishes (Platessa) die without ever reaching maturity, 

 and before the eyes have become asymmetrically placed ; and, in 

 the case of some of the rock-lobsters (Palinurus), the fiattened larvce 

 attain to gigantic proportions. Other instances of such abnormal 

 development might be mentioned. The unusually large size of the 

 eyes in some of the annelids and crustaceans (Alciops, Corycseus) 

 recalls a similar characteristic belonging to many of the deep-sea 

 forms of life ; likewise the total absence, or very rudimentary con- 

 dition (as in pteropods), of these organs. The power of emitting 

 phosphorescent light is another feature held in common by many of 

 the surface forms of life with the fauna of the deep. Pyrocystis 

 and Noctiluca, amoeboid bodies on the border-line between the 

 Foraminifera and Infusoria, appear to be the principal contributors 

 to the general oceanic phosphorescence. 



A remarkable feature of the pelagic fauna is the vast swarms 

 or schools in which many of the forms are found, association being 

 the rule rather than the exception, and the broad expanse over 

 which the greater number of the types are spread. The genera 

 are of almost universal distribution, and many even of the species, 

 of both the higher forms (fishes) and the lowest, are identical over 

 the most distantly removed quarters of the globe; the polar faunas, 

 however, wiiich are constituted principally by the crustaceans, 

 pteropods, and whales, differ materially from the faunas of the 

 temperate and equatorial belts, lacking largely in the medusa?, 

 the tunicates, and pelagic fishes. The varying salinity of the 

 oceanic waters appears very sensibly to affect this fauna, whose 

 distribution is, accordingly, in a measure governed by it. Thus, 

 the surface-fauna of the Baltic is very meagre, and in the upper 

 part of the basin, where the waters are nearly fresh, it is reduced 

 to little more than a very limited number of crustaceans. Some 

 of the medusoids, however, as Aurelia and Cyanea, appear to be 

 but little affected by a deficiency in the salt-supply, and, indeed, 



