36 MANUAL OF ZOOLOGY. 



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 Lemaese, both of which belong to 

 the Crustacea. 



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

 parasitic Lernsese the embryo is free-swimming and provided 

 with organs of vision and sensation, being in most 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 sedentary 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. 



12. Spontaneous Generation. 



Spontaneous or Equivocal generation is the term applied 

 to the alleged production of living beings without the pre- 

 existence of germs of any kind, and therefore without the 

 pre-existence of parent organisms. The question is one which 

 has been long and closely disputed, and is far from being 

 settled ; so that it will be sufficient to indicate the facts upon 

 which the theory rests. 



If an animal or vegetable substance be soaked in hot or 

 cold water, so as to make an organic infusion, and if this in- 

 fusion be exposed for a sufficient length of time to the air, the 

 following series of changes is usually observed : — 



1. At the end of a longer or shorter time, there forms upon 

 the surface of the infusion a thin scum, or pellicle, which, 

 when examined microscopically, is found to consist of an in- 

 calculable number of extremely minute molecules.' 



2. In the next stage these molecules appear, many of them, 

 to have' increased in size by endogenous division, till they form 

 short staff-shaped filaments, called " bacteria." These increase 

 in length by the same process until we get long filamentous 

 bodies produced, which are termed " vibriones."* Both the 

 bacteria and the vibrios now exhibit a vibratile or serpentine 

 movement through the surrounding fluid. 



3. After a varying period, the bacteria and vibrios become 



* By some authorities it is believed that the bacteria are produced by the 

 fusion together of the primitive molecules in twos and threes ; and that the 

 vibrios are produced out of the bacteria by the addition of fresh molecules 

 to the extremities of the latter, or by their uniting vi-ith one another. 



