Much in what has been adduced above may be erroneous, many an interpretation may have failed, and many a fact may not have been placed in its proper light. But in one thing, I hope, I have succeeded,—in convincing _unprejudiced_ readers, that Darwin’s theory furnishes the key of intelligibility for the developmental history of the Crustacea, as for so many other facts inexplicable without it. The deficiencies of this attempt, therefore, must not be laid to the charge of the plan drawn out by the sure hand of the master, but solely to the clumsiness of the workman, who did not know how to find the proper place for every portion of his material. [1] It is well known that, in many cases, even in adult animals the last segment of the middle-body, or some of its last segments, either want their limbs or are themselves deficient (_Entoniscus Porcellanæ_ male, _Leucifer,_ etc.). This might be due to the animals having separated from the common stem before these limbs were formed at all. But in those cases with which I am best acquainted, it seems to me more probable that the limbs have been subsequently lost again. That these particular limbs and segments are more easily lost than others is explained by the circumstance that, as the youngest, they have been less firmly fixed by long-continued inheritance. (“Mr. Dana believes, that in ordinary Crustaceans, the abortion of the segments with their appendages almost always takes place at the posterior end of the cephalothorax.”—Darwin, Balanidæ, page 111.) [2] _Persephone,_ a rare Crab, belonging to the family Leucosiidæ, is served in the same manner by its long chelate feet. If we seize the animal, it extends them most obstinately straight downwards, so that in all probability we should more easily break than bend them. [3] I must not omit remarking that what has been said as to the development of the Crabs applies essentially only to the groups _Cyclometopa, Catometopa_ and _Oxyrhyncha,_ placed together by Alph. Milne-Edwards as “Eustomés.” Among the _Oxystomata,_ as also among the “Anomura apterura,” Edw., which approach so nearly to the Crabs, I am unacquainted with the earliest young states of any of the species.