16 CRUSTACEA CHAP. 
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 ege is converted into the adult form. Crustacean 
eges 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, ae. 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, we. 
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 lmbs, often in two series (“ arthro- 
branchiae ”), 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. 
After this brief survey of Crustacean organisation we 
