188 Dynamic Theory. 



make its attachment to the log, the two eyes are degenerated into a sin- 

 gle minute eye spot, and the swimming legs now act to paddle the cur- 

 rent, with what food it may contain, toward the mouth. Both these 

 animals show the degradation brought about by the disuse of organs. 

 They are both related to the Cypris, and other branches of the very an- 

 cient Ostracoid family, some of whose members we found away back in 

 Silurian times. Their different degrees of degradation are brought 

 about by different degrees of disuse the old Barnacle being willing to 

 earn his living at a sedentary employment, while the Sacculina is un- 

 willing and, in fact, now unable to even digest its food, much less to 

 work for it. 



There is a large number of Crustaceans, some of them related to the 

 Sacculina, that lead such a life of parasitism as it does. A notable 

 case is the Lernean, one of the numerous tribe of fish-lice. It is at first 

 a six-legged Nauplius with a single eye, and lively habits, but fastening 

 upon the gill of a fish it settles down to parasitism and loses its shape, 

 its limbs and all its activities and functions, except to bear and mature 

 the eggs for the next generation. There are also many tribes that im- 

 itate the life of the Barnacle, beginning as lively and active animals, 

 with a well-developed nervous system, locomotive and sense organs, 

 they spend their young days in seeing the world, then settle down upon 

 ;some fixed object, lose their activity and everything superfluous in the 

 way of organs or senses, and merely vegetate during the rest of their 

 lives. The Balani, or Sea Acorns, live such a life. They are Crusta- 

 ceans and related to the Barnacles. The Ascidians, or Sea Squirts, are 

 another and a remarkable example (see Figs. 58 and 59). For the free 

 swimming tadpole-like larva possesses, along with other advanced marks 

 of development, a veritable notochord or rudiment of a backbone, such 

 as all vertebrate embryos develop at first. In the sedentary state which 

 follows this free life this notochord is abolished and with it a large part 

 of the nervous s} r stem, locomotive organ, &c. , proving the sea squirt to 

 be a degenerate family from a stock possessing a notochord and preserv- 

 ing their activity to the end of their days like our modern Amphioxus. 

 To this ancient stock, through which all vertebrates must trace their 

 pedigree, Hseckel has given the name Chordonia, or chorda animals. 



This mode of life in which the }^outh is locomotive, active and sensi- 

 tive, and the old age fixed, motionless and stolid, with a greater or less 

 abortion and degradation of organs and senses, is extremely common 

 amongst the mollusks and the plant animals, and is the rule among 

 the plants themselves, seeds generally possessing more or less ability to 

 be moved ; some having organs specially fitted to aid in their movable- 

 ness, as the wings of maple seed and the down of thistles and cotton- 

 wood, while others, as the spores of the protococcus, alga, &c. , have 



