The young animal, whilst still in the egg, acquires the full number of its segments and limbs. In cases where segments are amalgamated together, such as the last two segments of the thorax in _Dulichia,_ the last abdominal segments and the tail in _ Gammarus ambulans_ and _Corophium dentatum,_ n. sp., and the last abdominal segments and the tail in _Brachyscelus,_[4] or where one or more segments are deficient, as in _Dulichia_ and the _Caprellæ,_ we find the same fusion and the same deficiencies in young animals taken out of the brood-pouch of their mother. Even peculiarities in the structure of the limbs, so far as they are common to both sexes, are usually well-marked in the newly hatched young, so that the latter generally differ from their parents only by their stouter form, the smaller number of the antennal joints and olfactory filaments, and also of the setæ and teeth with which the body or feet are armed, and perhaps by the comparatively larger size of the secondary flagellum. An exception to this rule is presented by the Hyperinæ which usually live upon Acalephæ. In these the young and adults often have a remarkably different appearance; but even in these there is no new formation of body-segments and limbs, but only a gradual transformation of these parts.[5] Figs. 44 to 46. Feet of a half-grown Hyperia Martinezii, n. sp. (Named after my valued friend the amiable Spanish zoologist, M. Francisco de Paula Martinez y Saes, at present on a voyage round the world.) Figs. 47 to 49. Feet of a nearly adult male of the same species; 44 and 47 from the first pair of anterior feet (gnathopoda); 44 and 48 from the first, and 46 and 49 from the last pair of thoracic feet. Magnified. Figs. 44–46. Feet of a half-grown Hyperia Martinezii, n. sp. (Named after my valued friend the amiable Spanish zoologist, M. Francisco de Paula Martinez y Saes, at present on a voyage round the world.) Figs. 47–49. Feet of a nearly adult male of the same species; 44 and 47 from the first pair of anterior feet (gnathopoda); 44 and 48 from the first, and 46 and 49 from the last pair of thoracic feet. Magnified. Thus, in order to give a few examples, the powerful chelæ of the antepenultimate pair of feet, of _Phromina sedentaria,_ are produced, according to Pagenstecher, from simple feet of ordinary structure; and _vice versà,_ the chelæ on the penultimate pair of feet of the young _Brachyscelus,_ become converted into simple feet. In the young of the last-mentioned genus the long head is drawn out into a conical point and bears remarkably small eyes; in course of growth, the latter, as in most of the Hyperinæ, attain an enormous size, and almost entirely occupy the head, which then appears spherical, etc.