STRUCTURE OF THE AMAROUCIUM TADPOLE 85 



but it is coextensive with the muscle bands. It no doubt consti- 

 tutes the pathway for nerve fibers from the visceral ganglion 

 to the muscle cells, but the endings of fibers in muscle cells could 

 not be made out. 



DEFINITIVE GANGLION, HYPOPHYSIAL DUCT, AND SUBNEURAL 

 GLAND 



The parts of the nervous system described in the foregoing 

 paragraphs are those which function during the free-swimming 

 period of the tadpole and degenerate when the larval period is 

 over. The parts that persist and become the functional nervous 

 system of the sessile ascidiozooid are the hypophysial duct, 

 definitive ganglion and subneural gland. As shown in figure 

 D, they form a vertical series of structures situated immedi- 

 ately to the left of the sensory vesicle in the median sagittal 

 plane of the body. 



These structures -in their fully differentiated condition in 

 the adult ascidiozooid have been studied by Metcalf ('00). A 

 comparison of his figure 47 with figure D of this paper shows 

 that the entire central nervous sytem of the adult Amaroucium 

 ascidiozooid is fully formed in the larva, and thus shows clearly 

 the relation the larval nervous structures bear to those which 

 persist in the adult. 



The hypophysial duct is hollow and its canal is lined with 

 cilia for about two-thirds of its length (figs. C, D, and 1). At 

 its anterior end it is continuous with the wall of the oral siphon 

 and the cavities of these structures are in open communication. 

 As the ectodermal oral siphon at this stage is in no way connected 

 with the endodermal pharynx, there can be no doubt, in the case 

 of Amaroucium, of the primary connection of the hypophysial 

 duct with the ectodermal, and not with the endodermal part of 

 the alimentary tract — a fact of considerable significance for 

 the old controversial question of the homology of the hypophys- 

 ial duct of ascidians with the hypophysis of vertebrates. The 

 posterior end of the hypophysial duct terminates blindly in the 

 region of the atriopore between the lateral horns of the atrium. 

 The part of the duct which lies posterior to the subneural gland 

 corresponds to the rapheal duct of the ascidiozooid. 



