anterior bear an unusual abundance of olfactory filaments. Their youngest larvæ of course cannot swim; they are helpless little animals which firmly cling especially to the swimming laminæ of their host; the adult _ Hyperiæ,_ which are not unfrequently met with free in the sea, are, as is well known, the most admirable swimmers in their order. (“Il nage avec une rapidité extrême,” says Van Beneden of _H. Latreillii_ M.-Edw.) The transformation of the _Hyperiæ_ is evidently to be regarded as _acquired_ and not _ inherited,_ that is to say the late appearance of the abdominal appendages and the peculiar structure of the feet in the young are not to be brought into unison with the historical development of the Amphipoda, but to be placed to the account of the parasitic mode of life of the young. As in _Brachyscelus,_ free locomotion has been continued to the adult and not to the young, contrary to the usual method among parasites. Still more remarkable is a similar circumstance in _Caligus,_ among the parasitic Copepoda. The young animal, described by Burmeister as a peculiar genus, _ Chalimus,_ lies at anchor upon a fish by means of a cable springing from its forehead, and having its extremity firmly seated in the skin of the fish. When sexual maturity is attained, the cable is cut, and the adult _Caligi,_ which are admirable swimmers, are not unfrequently captured swimming freely in the sea. (See ‘Archiv. für Naturgeschichte’ 1852, I. p. 91). [6] “I know of no case in which the inferior (antennæ) are obsolete, when the superior are developed,” Dana. (Darwin, ‘Monograph on the Subclass Cirripedia, Lepadidæ’ p. 15.)