228 



THE POPULAR SCIENCE MONTHLY. 



generates to become a mere elongated worm, devoted to the produc- 

 tion of eggs, and exhibiting but little advance on the sacculina. 

 There are dozens of low crustaceans which, like sacculina, afford ex- 

 amples of animals which are free and locomotive in the days of their 

 youth, but which, losing eyes, legs, digestive system, and all the 

 ordinary belongings of animal life, "go to the bad," as a natural result 

 of participating in what has been well named "the vicious cycle of 

 parasitism." 



Plainly marked as are the foregoing cases, there are yet other 

 familiar crustaceans which, although not parasites, as a rule, neverthe- 

 less illustrate animal retrogression in an excellent manner. Such are 

 the sea-acorns (Halani), which stud the rocks by thousands at low- 

 water mark, and such are the barna- 

 cles (Fig. 12), that adhere to floating 

 timber and the sides of ships. In the 

 development of sea-acorns and bar- 

 nacles, the first stage is essentially 

 like that of the sacculina. The young 

 barnacle is a " nauplius," three-legged, 

 free-swimming, single-eyed, and pos- 

 sessing a mouth and digestive appa- 

 ratus. In the next stage we again 

 meet with the six pairs of swimming- 

 feet seen in sacculina, with the enor- 

 mously developed front pair of legs 

 serving as "feelers," and with two 

 " magnificent compound eyes," as Dar- 

 win describes the organs of vision. 

 ^ The mouth in this second stage, how- 



nSsy ^^mil n ever, is closed, and feeding is there- 



m^ ^-^-mUJ ^j,g impossible. As Darwin remarks. 



Fig. 12. Barnacles. ^^ function of the young barnacles 



"at this stage is to search out by 

 their well-developed organs of sense and to reach by their active 

 powers of swimming a proper place on which to become attached, 

 and to undergo their final metamorphosis. When this is com- 

 pleted," adds Darwin, " they are fixed for life ; their legs are now 

 converted into prehensile organs ; they again obtain a well-constructed 

 mouth, but they have no antennae, and their two eyes are now 

 reconverted into a minute, single, simple eye-spot." A barnacle is 

 thus simply a highly modified crab-like animal which fixes itself by its 

 head to the floating log, and which " kicks its food into its mouth 

 with its feet," to use the simile and description of biological authority. 

 The development of its "shell" and stalk are matters which do not in 

 the least concern its place in the animal series. These latter are local 

 and personal features of the barnacle tribe. For in the " sea-acorns," 



^f>///. 



