453 



CRUSTACEA. 



(Asellus) or they unite together into a common median penis which 

 lies at the base of the abdomen (Oniscidce). A pair of styliform or 

 complicated, hook-bearing appendages of the anterior abdominal feet 

 are to be looked upon as accessory copulatory organs \ in addition 

 to these a pair of outwardly turned chitinous rods on the inner side 

 of the second pair of feet may also be present (Oniscidce). The 

 Cymothoidce are hermaphrodite* (Bullar), but the sexual organs 

 become ripe at different times. In the young stage these animals 

 function as males, and possess three pairs of testes, two rudimentary 

 ovaries internal to the testes, and a paired copulatory organ into 



which the two vasa deferentia 

 ^ open (fig. 360). After a subse- 



quent ecdysis and after the fe- 

 male glands have developed at 

 the expense of the gradually 

 diminishing male glands, the 

 oostegites, which in the meantime 

 have been developed, become free 

 on the thoracic legs and the copu- 

 latory organs are thrown off. 

 Henceforward the animal func- 

 tions only as a female. 



The embryonic development 

 begins after the entry of the eggs 

 into the brood pouch and is in- 

 troduced by a centro-lecithal seg- 

 mentation, the central part of 

 the egg (food yolk) remaining at 

 first unsegmented. The blasto- 

 derm soon consists of a periphe- 

 ral layer of naked nucleated cells 

 and produces by a rapid growth 

 of its constituent cells the ventrally placed germinal bands, at the 

 anterior end of which the cephalic lobes are first marked off. The 

 rudiments of the trifoliate appendages (dorsal organ) of the Isopod 

 embryos are next formed as two prominences on the cephalic lobes. 

 The physiological and morphological meaning of these structures has 

 not yet been explained. Of the appendages the two pairs of antennae 



* J. Bullar, " The generative organs of the Parasitic Isopoda," Jovrn. Anat. 

 Pky?iol.. 1876. P. Mayer, "Ueber den Hermaphroditismus einiger Isopoden," 

 Mittheil. aus der Zool. Stat. Nt>apcl, 1879. 



FIG. 300. a, S emale of Cymothoa Amiti (after 

 M. Edwards). Brl, oostegite. b, Sexual 

 organs from a Cymothoa cestridet, 13 mm. in 

 length (after P. Mayer). T, The three 

 testes ; Oo, ovary ; Od, oviduct j Vd, vaa 

 def erens ; P, penis. 



