SPONTANEOUS GENERATION. 43 



vide for their complete development within her own sub- 

 stance are rapidly hatched, give birth to imperfect offspring, 

 which, in proceeding to their definitive characters, undergo 

 several alterations in structure and form, known as metamor- 

 phoses." 



Retrograde Development. Ordinarily speaking, the course 

 of development is an ascending one, and the adult is more 

 highly organised than the young ; but there are cases in which 

 there is an apparent reversal of this law, and the adult is 

 to all appearance a degraded form when compared with the 

 embryo. This phenomenon is known as " retrograde " or 

 "recurrent" development; and well-marked instances are 

 found amongst the Cirripedia and Lernaeae, both of which 

 belong to the Crustacea. 



Thus, in the Cirripedes (acorn - shells, &c.) and in the 

 parasitic Lernaeae (fig. 3), the embryo is free-swimming and 



Fig. 3. A, Young of one of the parasitic Crustaceans (Achtheres) with its swimming- 

 paddles and eye-spots, magnified ; B, Deformed and swollen parasitic adult of an- 

 other member of the same group (Lerncea). 



provided with organs of vision and sensation, being in many 

 respects similar to the permanent condition of certain other 

 Crustacea, such as the Copepods. The adult, however, in 

 both cases, is degraded into a more or less completely seden- 

 tary animal, more or less entirely deprived of organs of sense, 

 and leading an almost vegetative life. As a compensation, 

 reproductive organs are developed in the adult, and it is in 

 this respect superior to the locomotive, but sexless, larva. 



13. SPONTANEOUS GENERATION. 



Spontaneous or Equivocal generation is the term applied to 

 the alleged production of living beings without the pre-exist- 

 ence of germs of any kind, and therefore without the pre-exist- 

 ence of parent organisms. The question is one which has 



