466 



CEUSTACEA. 



filaments and hairs on the surface of the anterior antennae have the 

 value of olfactory organs; the antennas function as tactile or gam, 

 as do also the palps of the jaws, the maxillipeds and the legs. 



The generative organs are paired and lie in the thorax or in the 

 abdomen (Stomatopoda}, and, as a rule, are connected across the 

 middle line by a median portion. The female organs consist of two 

 ovaries and two oviducts, which open on the basal joint of the antepen- 

 ultimate pair of ambulatory legs or on the sternal region between 

 these appendages (fig. 365, a). The testes (fig. 365, b) are composed 

 of numerous sacs and blind tubes, and, like the ovaries, are connected 

 by a median portion; there are two vasa deferentia, often much 

 coiled, which open on the basal joint of the last pair of ambulatory 

 legs, more rarely on the sternum, and occasionally on a special 



copulatory organ (Schi- 

 zojwda). The first, or 

 the first and second, 

 pair of abdominal feet 

 act as intromittent or- 

 gans. The eggs either 



FIG. 866. Crab zoeea (Thia), after the first moult. ZS, 

 Zoa-a spine on the back ; Kf, Kf 1 ', the two pairs of 

 "biramous appendages corresponding to the first and 

 second pairs of maxillipeds. 



pass into a brood-pouch 

 formed by lamellar ap- 

 pendages of the thoracic 

 legs (Cumacea, ScMzo- 

 poda\ or become at- 

 tached by means of the 

 cementing secretion of 

 special glands to the 

 hairy abdominal feet of 

 the female, where they remain until they are hatched (Deccqwdci). 



Development. Most of the Thoracostraca undergo a metamor- 

 phosis which may be more or less complicated. The Cumacea, some 

 ticldzopoda (Mysidea} and the fresh- water Decapoda (Astacus) leave 

 the egg membranes with the fall number of segments and appen- 

 dages. All the Stomatopoda, on the contrary, as well as most of the 

 Decapoda, are hatched as larvae ; the latter in the so-called Zocea 

 form with only seven pairs of appendages in the anterior region of 

 the body (there are two pairs of antenna?, mandibles, two pairs of 

 maxilla?, and two pairs of maxillipeds), without the last six thoracic 

 segments and with a long abdomen destitute of appendages (fig. 366). 

 The two pairs of antenna? of the Zocea are short and destitute of 

 flagella. The mandibles are without a palp ; the maxillae are already 



