GONADS. OVA. TORN ARIA. 95 



In Stereobalanus canadensis there are several rows of gonads, both inside 

 and outside the branchial pores. Their openings are all in the submedian 

 line which is much widened out. In Ptychodera there are usually several 

 rows of accessory gonads which are for the most part external to the primary 

 gonads. 



The extension of the genital organs varies in the different species. In 

 some species (e.g. Ptychodera flava) they are found throughout the branchio- 

 genital region, beginning just behind the collar ; in other species (e.g. 

 Glandiceps abyssicola) they are confined to that part of the branchio -genital 

 region which lies behind the branchial pores ; while in Stereobalanus cana- 

 densis they are coextensive with the branchial region. In the majority of 

 species, however, they begin in the branchial region at a variable distance 

 behind the collar. In almost all cases the branchio-genital region over- 

 laps the hepatic region, so that genital sacs are found at the anterior 

 part of the latter. In Bal. carnosus, however, the transition is abrupt and 

 no genital sacs are found in the hepatic region. 



In most of the Ptychoderidse the side walls of the body in the 

 branchiogenital region are prolonged into winglike folds the 

 genital pleurae. The genital sacs are for the most part con- 

 tained in these wing-like outgrowths, but not entirely, for in 

 some species they are found in the body as well, mediad 

 of the insertion of the lateral septum. The genital pleune are 

 extremely mobile structures and may be bent up so as to cover 

 the back of the animal. In Bal. carnosus their free opposed 

 edges may become united by mucus, so that a cavity, widely 

 open behind and receiving the branchial pores, is formed on the 

 dorsal side of the branchiogenital region. 



The gametes are produced from the walls of the gonads and 

 when ripe are passed out to the exterior through the genital 

 pore's. The spermatozoa have spherical or oval heads and 

 active flagelliform tails. The ova are provided with a close- 

 fitting egg-membrane, and are of two sizes according to the 

 amount of yolk and manner of development. 



In most genera the egg is small, varying from *06 mm. in diame- 

 ter in Ptychodera flava to 15 mm. in Bal. carnosus. The develop- 

 ment of such eggs is probably indirect, passing through a free 

 larval stage, called the Tornaria (Fig. 74). In some cases, 

 however, the eggs are much larger, varying in their longest 

 diameter from '4 mm. in Dolichoglossus Tcowalevslcii to 1*5 in 

 Harrimania kupfferi. 



The early stages of the small eggs which probably develop 

 into tornaria are not known, but thanks to the researches of 

 Bateson all the stages of the development of Dolichoglossus 



