352 CHAPTERS ON EVOLUTION. 



pairs of swimming feet are cast off, the body assumes its sac-like 

 appearance, and the sacculina sinks into its adult stage a pure 

 example of degradation by habit, use, and wont. So also with 

 certain near neighbours of these crab-parasites, such as the Lerneans, 

 which adhere to the gills of fishes. Beginning life as a three-legged 

 "nauplius," the lernean retrogresses and degenerates to become a 

 mere elongated worm, devoted to the production of eggs, and ex- 

 hibiting but little advance on the sacculina. There are dozens of 

 low crustaceans which, like sacculina, afford examples 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, never- 

 theless illustrate animal retrogression in an excellent manner. Such 

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

 water mark, and such are the barnacles (Fig. 252), that adhere to 

 floating timber and the sides of ships. In the development of sea- 

 acorns and barnacles, the first stage is essentially like that of the 

 sacculina. The young barnacle, as our previous studies have shown, 

 is a " nauplius," three-legged, free- swimming, single-eyed, and possess- 

 ing a mouth and digestive apparatus. 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 Darwin describes the organs of vision. 

 The mouth in this second stage, however, is closed, and feeding is there- 

 fore impossible. As Darwin remarks, the function of the young bar- 

 nacles "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 metamor- 

 phosis. When this is completed," 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," which pass through an essentially similar develop- 

 ment, there is no stalk; and the animal, after its free-swimming stage, 

 simply glues its head, by a kind of marine cement of its own manu- 

 facture, to the rock, develops its conical shell, and like the barnacle 



