166 PRINCIPLES OF ANIMAL BIOLOGY 



needle and moth eggs raised to a high temperature have yielded adult 

 offspring. 



Paedogenesis. — Although sexual reproduction is usually carried on 

 only by adults, this is not always the case, for there are certain species 

 whose members have the remarkable power of reproducing sexually while 

 they are in the larval ^ state. This reproduction by a larval animal 

 is called -paedogenesis. Paedogenesis may be either parthenogenetic or 

 bisexual. 



Parthenogenetic paedogenesis occurs in certain species of flies. The 

 larvae in these species (Fig. 134) produce ova which develop by partheno- 

 genesis into larvae before the oviducts are present. The latter generation 

 of larvae escapes from the parent larva by rupture of the body wall. This 

 results in the death of the parent. Several generations may be produced 



in this fashion; then the larvae of one 

 generation pupate and emerge as normal 

 adult male and female flies. 



Fig. 134. — Paedogenesis in the Paedogenesis of the bisexual type 



fly Miastor. The parent, itself a -iU iii lii/ii. 



larva, contains a number of larval OCCUrS m the Wcll-knOWn axolotl Amhy- 



ofTspring. {From Folsom after stoTYia Hgrinum, ov tiger Salamander. 



Pagenstecher.) tt i j. • j-i- au* • i 



Under certam conditions this animal 

 attains sexual maturity and breeds while it is still in the larval form 

 having gills. In some of the Mexican lakes this is said to be the 

 usual occurrence, while in Kansas and Nebraska it is rare, and in many 

 localities it probably does not occur at all. 



Hermaphroditism. — Most animals — a very great majority of the 

 metazoa — possess either male or female organs of reproduction but not 

 both. Species which have the sexes thus separate are said to be 

 dioecious (living in two houses), while those species whose individuals 

 produce both eggs and spermatozoa are called monoecious (living in one 

 house). Individuals with both male and female organs are said to be 

 hermaphrodites.'^ Two common species of Hydra are hermaphroditic, 

 as are most of the flatworms, most snails, and some roundworms. In 

 many monoecious species the spermatozoa are produced first and later 

 the ova, but in some species this condition is reversed. By developing the 

 sexual products at different times, cross-fertilization, that is, fertilization 

 of eggs by spermatozoa from another individual, is assured. In the earth- 

 worm, eggs and spermatozoa are produced in the same individual and 



' A larva is a young independent individual which differs from the adult in the 

 possession of organs not possessed by the adult, or in lacking certain organs which arc 

 present in the adult (for example, a frog tad{K)le). 



"^ The word monoecious is also applied to individuals, and is then synonymous 

 with hermaphrodite; but the corresponding word dioecious cannot well be applied 

 to individuals. 



