ANATOMY OF ARENICOLA ASSIMILIS. 747 



Thu first nepliridium is usually distinctly smaller than any 

 of the others, a condition frequently noticed in A. marina. 

 Although this nephridium possesses a gonidial vessel, no 

 gonad is developed upon it. 



Gonads. — The gonads are, as in other species, associated 

 Avith the nephridia, and are present on all except the first 

 pair. Each gonad is a club-shaped mass of cells about I'O 

 mm. long (fig. 17), formed by proliferation of the cells 

 covering the gonidial vessel (Gamble and Ashworth, 1900, 

 p. 521) immediately behind the nephrostome. The forma- 

 tion of ova and spermatozoa follows the same course as in 

 A. marina. The ova present in the coelomic fluid of the 

 specimen from Punta Arenas are appareutly mature,^ and 

 have a distinct but thin vitelline membrane (3 /.t thick). 

 They are not spherical, but somewhat discoidal. The face of 

 the disc is usually oval, and measures 0'19 to 0"20 mm. by 0"15 

 to 0*16 mm. in diameter. The thickness of the egg is about 

 0"075 mm. Measurements of a considerable number of well- 

 preserved unshrunk eggs from the coelomic fluid show that 

 the three axes above named are fairly constant in propor- 

 tion. It will be convenient to correct here a statement in 

 the memoir by Dr. Gamble and myself (1900, p. 527) in 

 which the ova of A. marina, A. claparedii, and A. 

 cristata are described as spherical. This is a mistake, as the 

 ova of all these species are flattened in one plane, like those of 

 A. assimilis described above. In A. marina"-^ the face of 

 the egg is either circular (usually) and about 0"14to0T5 mm. 

 in diameter, or it is oval, Avith diameters of 0"16 and 0'12 to 

 0'14 mm., and the third axis of the egg is from 0"08 to 0'09 

 mm. in length. The ova of A. claparedii are usually only 

 slightly oval, the two diameters of the face of the egg being 

 about 0'16 mm. and 0"14 to 0*16 mm. respectively, and the 



' Tills spccinicn was taken in September, 1SU:2. 



•i Tlie folluwiiij^ deseripUoii and. nieasurenicnls may replace lliose given on 

 p. 527 in the memoir cited aljove ; they have been drawn either hum Hving or 

 well-preserved ova, most of whieli have come into my hands since tlie com- 

 pletion of that memoir. 



