METAMORPHOSIS. 939 



in each single individual i. e., a transformation takes place not in 

 the embryo in ovo but after the escape of the young from the ovum as 

 an independent being. In such instances, the young animal, on 

 emerging from a fertilized ovum, has at first no resemblance to the 

 parent, but exhibits a provisional form and organization, suited to its 

 conditions of life. After a time, however, it undergoes changes ; some 

 organs or parts disappear, whilst others begin to be formed, and, 

 finally, it assumes a state of mature existence, resembling its parent, 

 and exhibiting one or other form of sex. Thus the larva, or worm- 

 like caterpillar, of the Insect, proceeds from the egg. Consuming 

 large quantities of food, it grows, and then changes into the pupa, or 

 chrysalis ; in this condition, no food being taken, remarkable changes 

 occur, of which the formation of wings is the most obvious. From 

 this, finally, it emerges as the imago or perfect insect. The relative 

 degree or extent of the metamorphosis, differs in different Orders and 

 Families of Insects. The suspension or arrest of the ordinary phases 

 of this metamorphosis, occasionally gives rise to monstrosities, such as 

 butterflies with caterpillars' heads, and other curious forms. 



Metamorphosis may also be said to occur in some of the lowest 

 Fishes, certain forms of Ammocete having been shown to be the larvae 

 of the lamprey, which afterwards undergo comparatively slight changes 

 in the buccal and branchial apparatus. 



In the Amphibia, the tadpoles of the "frogs and others, as developed 

 from the egg, present a fish-like form, and possess at first external 

 and then internal gills ; but ultimately, in 'the higher forms, they as- 

 sume the perfect Batrachian conformation, lose their gills, and breathe 

 by lungs. The extent of change is most marked in the anourous or 

 tailless Amphibia. In the salamanders, however, no internal gills are 

 developed, like those of the frog ; and the tail, instead of undergoing 

 interstitial absorption, is retained. The suspension of the metamor- 

 phic process, at certain early stages, leads to the formation of the Pe- 

 rennibranchiate Amphibia, in which lungs also exist, such as the Pro- 

 teus, Siren, Axolotl, and Menobranchus. 



The preceding cases are instances of progressive metamorphosis. 

 But metamorphosis may be, as far as general organization is con- 

 cerned, retrograde, animals being, in the larval stage, actively loco- 

 motive, and, in the perfect stage, fixed or sessile. Thus, the young 

 of the Ascidioida are free-swimming, tailed, and ciliated animals, 

 whilst in their perfect condition they are fixed. In the Crustacea, the 

 larvae exhibit progressive metamorphoses of a remarkable character. 

 In the Cirrhopods, the larvae are active, move freely in the water, and 

 possess eyes, but, afterwards, they become sessile, fixed by the head, 

 and lose those organs. They present an example of retrograde or re- 

 current metamorphosis. In the parasitical Crustacean Lerneada, which 

 attach themselves to fishes, and even in the Lamellibranchiate Mol- 

 lusca, the perfect animal is less highly endowed than when in its larval 

 condition. 



The phenomena of individual metamorphosis, so obvious in the. In- 

 sects and the Amphibia, after their escape from the egg, are in re- 

 ality, not singular ; for phases of evolution or transformation, occur 



