1 6 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 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 Anaspidcs, p. 116) carried about by the female either in 

 a brood-pouch (Branchiopoda, Ostracoda, Cirripedia, Phyllocaridii, 

 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 

 dose 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 (" 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 ]ioilol,n-;inch, two arthrobranchs, and a pleurobranch, 

 but the full complement of gills is never present, various 

 nifiiihrrs <>f 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 



