In the course of the development of this entozoon, Leuckart recognises four well-marked stages. The _first_ is that of the embryo with a boring apparatus. In the _second_ stage, the embryo has become transformed into a motionless pupa. The _third_ is the ordinary larval condition characterised by numerous rows of small spines in addition to two pairs of double claws. The _fourth_ is the sexually-developed stage, furnished with a simple hook-apparatus, and without integumentary denticles. “Our Pentastomes, therefore,” says Leuckart, “exhibit two kinds of larval forms, an earlier and later one, such as takes place in other animals; this also occurs even in insects (_Strepsiptera_ and _Meloidæ_), only that, in our case (_i. e._ in _Pentastoma_), both do not immediately follow one another, but are separated by a resting condition, which I have designated as the pupa stage. In choosing this name I do not mean to express a complete identity of this intermediate state with the pupal sleep of insects.” [Illustration: FIG. 51.--Upper third of the body of _Pentastoma denticulatum_. Original.] So far as my own observations extend, the pupa, in its later stages, closely resembles the free larva; but, as Leuckart points out, the earlier stages are very different. The embryo, after encystation, repeatedly casts its skin, and during the intervals of these several successive moultings, the young animal makes rapid growth, accompanied by a series of structural changes. Passing through these it at length acquires the perfected larval state (_P. denticulatum_).