376 GOMPARATIVE ANATOMY chap. 



pass into the oviducts by the bursting of their walls, aud thus bring about the 

 fertilisation of the eggs in the ovaries. The animal then casts its skin, and with 

 the skin the receptacula seminis, so that the two genital apertures are no longer 

 present. The fertilised eggs pass from the ovarium into the body cavity and thence 

 through a newly -formed unpaired birth aperture in the last thoracic segment but one 

 into the brood cavity. A new batch of eggs is fertilised later in the ovary by sperm 

 left over from the first copulation, and this reaches the brood cavity in the same 

 way. After this second batch of eggs has been developed in the brood cavity and 

 the young that are hatched have left it, the animal moults, and then again appears 

 as it was before copulation. > 



Thoracostraca. — The genital organs of the Cumacea and in some respects also 

 those of the Schizopoda need more thorough investigation. The paired sexual glands 

 of the Thoracostraca are united by an unpaired piece which lies in the thorax in the 

 Decapoda and Schizopoda, and in the telson in the Stomatopoda (Fig. 250), and always 

 above the intestine. This piece is wanting only in the Ctimacea C^) and. Paguridm. 

 Except in the Stomatopoda and Pagurida: where the ovaries and testes lie in the 

 abdomen, the germ glands are entirely or for the greater part restricted to the thorax. 

 They everywhere lie between the intestine and the heart. The testis on each side is 

 a tube which either remains straight {Stomatopoda), or is coiled up in a complicated 

 manner {e.g. in Paguristes, Carcinus), or is provided with simple lateral invaginations 

 {Palinurus), or is much branched and carries small sacs at the ends of the branches 

 (Astacus). It is enclosed in an envelope of connective tissue. The long and much 

 coiled vasa deferentia run, like the oviducts in the female, posteriorly where the 

 germ glands are in the thorax, and anteriorly where they are in the abdomen. 

 They fall into two divisions, a proximal portion lying near the testes and a distal 

 glandular portion often provided with small invaginations, this latter division being 

 continued into the strongly muscular ductus ejaculatorius. This opens outwardly 

 at the basal joint of the last pair of thoracic feet either on a slight -swelling 

 {Macrura) or at the point of an elongated tubular penis (BracMura, Schizopoda), 



In the Stomatopoda there emerges at the point of the penis, besides the ductus 

 ejaculatorius, the duct of a paired tubular accessory gland (Fig. 250, A, d) which lies 

 in the free thoracic segments, and the two parts of which are connected anteriorly 

 by an unpaired intermediate piece. 



The ovaries agree in general with the testes in position and shape, but they are 

 simpler inasmuch as they are simple tubes or vesicles. In Squilla they have seg- 

 mental bulgings. The oviducts are shorter and not so much coiled as the sperm 

 ducts. They emerge at the typical point in the antepenultimate thoracic segment, 

 in Squilla immediately at the side of a median receptaculum seminis. 



We cannot here enter on the subject of egg and sperm formation in the Crus- 

 tacea. But the egg formation in the Cladocera, as it is peculiarly interesting, 

 must be briefly described. Successive groups, each consisting of four germ cells, 

 sever themselves from the germ layer when the production of summer eggs takes 

 place, but only one cell out of each group becomes an egg, the others being used up 

 as nourishment. In the production of winter eggs, however, only one cell out of 

 every second group of germ cells becomes an egg, while the remaining 7 cells of 

 the two groups serve as nourishment for the one egg. 



XI. Sexual Dimorphism. 



This is more or less marked in all Crustacea. There are indeed no outer or 

 inner portions of the body which in some one species, genus, or order of Crustacea 

 are not differently constructed in the two sexes, and these differences have great 

 biological significance and are of great importance in classification. We can here 

 only select the most important and most widely distributed. 



