MICROSCOPIC STUDY OF LOWER ANIMALS. 51 



characters they bear such a resemblance to Mollusks, that the 

 Barnacles which attach themselves to floating timber, and the 

 Acorn-shells which incrust the surfaces of rocks, are unhesitat- 

 ingly ranked by Shell-collectors among their " multivalves," yet 

 the close resemblance which exists between their early forms and 

 the little "Water-Fleas which swarm in our pools, makes it quite 

 certain that the Barnacles not only belong to the Articulated in- 

 stead of to the Molluscous series, but that they must be ranked in 

 close proximity to the Entomostracous division of the Crustacea, 

 if not actually as members of it. To the same discoverer, moreover, 

 we owe the knowledge that even the common Crab undergoes 

 metamorphoses scarcely less strange, its earliest form being a lit- 

 tle creature of most grotesque shape, which had been previously 

 described as an adult and perfect Entomostracan ; so that, al- 

 though scarcely any two creatures can apparently be more unlike 

 than a Barnacle and a Crab, they have (so to speak) the same 

 starting-point ; the difference in their ultimate aspect chiefly 

 arising from the difference in the proportionate development of 

 parts which are common to both. A still more remarkable series 

 of Metamorphoses has recently been shown by Prof. Muller to 

 exist among the Echinoderms (Star-fish, Sea-urchins, &c.) ; whose 

 development he has studied with great perseverance and sagacity. 

 Thus the larva of the Star-fish is an active free-swimming animal, 

 having a long body with six slender arms on each side, from one 

 end of which the young star-fish is (so to speak) budded off; and 

 when this has attained a certain stage of development, the long 

 twelve-armed body separates from it and dies oft', its chief func- 

 tion having apparently been, to carry the young Star-fish to a 

 distance from its fellows, and thus to prevent overcrowding by 

 the accumulation of individuals in particular spots, which would 

 be liable to occur if they never had any more active powers of 

 locomotion than they possess in their adult state. Scarcely less 

 remarkable are the changes which are to be witnessed in the 

 greater number of aquatic MollusJcs, almost all of which, however 

 inert in their adult condition, possess active powers of locomotion 

 in their larval state ; some being propelled by the vibratile move- 

 ment of cilia disposed upon the head somewhat after the fashion 

 of those of Wheel-animalcules, and others by the lateral strokes 

 of a sort of tail which afterwards disappears like that of a tadpole. 

 Among the Annelids or marine worms, again, there is found 

 to be an extraordinary dissimilarity, though of a somewhat dif- 

 ferent nature, between the larval and the adult forms : for they 

 commonly come forth from the egg in a condition but little ad- 

 vanced beyond that of Animalcules ; and, although they do not 

 undergo any metamorphosis comparable to that of Insects, they 

 pass through a long series of phases of development (chiefly 

 consisting in the successive production of new joints or segments, 

 and of the organs appertaining to these) before they acquire their 

 complete type. In nearly all the foregoing cases it may be re- 



