208 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



OVT U^lL 



nc. \*.--. 



/£__ 



(those which reach a definitive length of about 4 cm. or less) the 

 majority of zooids are protogynous. The larger forms, on the con- 

 trary, contain a large number of protandrous zooids, especially 

 during their period of active growth. This condition seems to 

 result from the fact that, in actively budding zooids, the nutriment 

 demanded by the maturing sex cells is diverted for the formation 

 of buds; hence the development of the egg, which requires a large 

 amount'ofjnutriment, is delayed. The testis is an aggregate of lobes 



forming an organ of somewhat 

 hemispherical form, either quite 

 compact (pi. 23, figs. 17 and 18), 

 or, in other species, rather open 

 (fig. 22, pi. 24), its lobes forming a 

 sort of rosette. There are a varia- 

 ble number of lobes, from 12 to 20 

 in the majority of forms, but there 

 may be as many as 77 in P. spino- 

 sum. These lobes converge on the 

 ventral side (dorsal side in the Py- 

 rosomata fixata) from which point 

 the sperm duct arises and leads to 

 the cloaca (fig. 22, pi. 24). The 

 single egg lies in an ovarian vesicle 

 formed by the outer and inner epi- 

 thelia of the cloacal wall. It con- 

 tains much yolk and is surrounded 

 by a follicular membrane. In con- 

 nection with the young egg is a 

 so-called oviduct, a narrow duct 

 dilated next to the egg, which runs 

 forward and opens into the cloaca. 

 fig. 5.— pyrosoma atlanticum paradoxum:.4, It serves as a sperm receptacle. 

 ovary and oviduct with two spermatozoa in After sperms have entered it, the 



THE OVIDUCT. B, TWO SPERMATOZOA. , j , L 



aperture grows shut and the sper- 

 matozoa are retained until the egg is ready for fertilization (fig. 5). 



The embryology of Pyrosoma is complicated, owing, first, to the 

 large accumulation of yolk in the egg, which distorts the early stages of 

 development; second, to the very early appearance of budding; and, 

 third, to the fact that the cyathozooid degenerates before reaching 

 fully adult structure. Cleavage is incomplete and discoidal. There 

 are formed by meridional division two, four, and then eight cells, 

 which continue to divide more irregularly, leading to the formation of 

 a flat germinal disk. The embryo develops from a central, thickened 



od.-- 



