The spermatozoa show a great variety of structure, but they conform to two chief types—the filiform, which are provided with a long whip-like flagellum; and the amoeboid, which are furnished with radiating pseudopodia, and are much slower in their movements. The amoeboid spermatozoa of some of the Decapoda contain in the cell-body a peculiar chitinous capsule, and Koltzoff[12] has observed that when the spermatozoon has settled upon the surface of the egg the chitinous capsule becomes suddenly exceedingly hygroscopic, swells up, and explodes, driving the head of the spermatozoon into the egg. We cannot enter here into a description of the embryological changes by which the egg is converted into the adult form. Crustacean eggs as a whole contain a large quantity of yolk, but in some forms total segmentation occurs in the early stages, which is converted later into the pyramidal type, _i.e._ the blastomeres are arranged round the edge, and the yolk in the centre is only partly segmented to correspond with them. The eggs during the early stages of development are in almost all cases (except Branchiura, p. 77, and _Anaspides_, p. 116) carried about by the female either in a brood-pouch (Branchiopoda, Ostracoda, Cirripedia, Phyllocarida, Peracarida), or agglutinated to the hind legs or some other part of the body (Copepoda, Eucarida), or in a chamber formed from the maxillipedes (Stomatopoda). Development may be direct, without a complicated metamorphosis, or indirect, the larva hatching out in a form totally different to the adult state, and attaining the latter by a series of transformations and moults. The various larval forms will be described under the headings of the several orders. The =respiratory organs= are typically branchiae, _i.e._ branched filamentous or foliaceous processes of the body-surface through which the blood circulates, and is brought into close relation with the oxygen dissolved in the water. In most of the smaller Entomostraca no special branchiae are present, the interchange of gases taking place over the whole body-surface; but in the Malacostraca the gills may reach a high degree of specialisation. They are usually attached to the bases of the thoracic limbs (“podobranchiae”), to the body-wall at the bases of these limbs, often in two series (“arthrobranchiae”), and to the body-wall some way above the limb-articulations (“pleurobranchiae”). In an ideal scheme each thoracic appendage beginning with the first maxillipede would possess a podobranch, two arthrobranchs, and a pleurobranch, but the full complement of gills is never present, various members of the series being suppressed in the various orders, and thus giving rise to “branchial formulae” typical of the different groups.