TSOPODA. 483 



In the parasitic Hemioniscus and Cryptomscus the alimentary 

 canal ends blindly in the adult state, in a bilobed sac produced 

 by the dilatation of two of the hepatic tubes. 



Excretory organs. A small antennary gland has been recog- 

 nized in the base of the first antenna, and a shell gland in the 

 maxillary region. 



The heart differs from that of Amphipods in its more posterior 

 position, being largely contained in the abdomen, in ending 

 blindly behind, and, in many cases at any rate, in giving off 

 lateral arteries direct to the limbs. 



Reproduction. In the great majority of Isopods as in most 

 other Crustacea, the sexes are separate, and in shape and 

 size the male and female are alike, but in association with the 

 parasitic, and hence more or less stationary habit, we meet with 

 two departures from the rule hermaphroditism, and strongly 

 marked sexual dimorphism. In the usual condition three tubular 

 lobes of the testis on either side of the male unite to form a 

 dilated seminal vesicle from which the vas deferens passes to a 

 cylindrical or papilliform paired (Asellus) or single (Oniscus) 

 penis, at or near the middle line on the last thoracic segment. 

 The second abdominal appendages of the male are, moreover, 

 provided with styliform processes from their inner borders. 



In the female the simple tubular ovaries communicate laterally 

 with oviducts which open on the sixth thoracic segment. Ooste- 

 gites, arising close to the bases of the limbs of from two to six 

 of the thoracic segments, form a brood pouch. 



The female genital apertures often appear only at the time of 

 formation of the brood pouch. 



The phenomena of fertilization and the production of the brood in the 

 Oniscidae, as described by Schobl * are very remarkable. According to 

 this author the two receptacula, which are imaginations of the outer 

 integument into the mouths of the oviducts, are at first not in 

 open communication with the latter. Only after sperm has, during 

 copulation, entered the receptacula, does it enter the oviducts 

 by the bursting of the walls separating them from the receptacula 

 and thus bring about the fertilization of the eggs in the ovaries. 

 The animal then casts its skin, and with the skin the receptacula 

 seminis. The two genital apertures are now no longer present. The 

 fertilized eggs pass from the ovaries into the body cavity and thence 

 through a newly-formed, unpaired, birth aperture in the last thoracic 



* J. Schobl, Ueb. d. Fortpflanzung isopoder Crustaceen, Arch. f. mikr. 

 Anat., Bd. xvii (1880), p. 125. 



