XIIE 



PHYLUM CHORDATA 



51 



dorsal and ventral fins containing the fin-rays (Fig. 753, fh.), and 

 paired canals in the metapleures (of.). 



Excretory Organs. The principal organs of excretion are 

 about ninety pairs of peculiarly modified nephridia (Fig. 752, neph.) 

 situated above the pharynx and in relation with the main crelomic 

 cavities. Each nephridium (Fig. 755) is a bent tube consisting 

 of an anterior vertical and a posterior horizontal limb. The 

 vertical limb terminates in a large group of solenocytes (Vol. I., 

 p. 468), and there are several smaller groups on the horizontal 

 limb. The organ 

 thus closely corre- 

 sponds to the type 

 of nephridium with 

 closed inner end 

 bearing s o 1 e n o- 

 cytes already de- 

 scribed as occur- 

 ring in certain of 

 the Polychaeta. 

 On the ventral 

 surface of the 

 horizontal limb, 

 opposite a secon- 

 d a r y branchial 

 lamella, is a single 

 aperture bearing 

 long cilia and 

 opening into the 

 atrium : this corre- 

 sponds with the 

 nephridiopore or 

 external aperture 

 of the typical 

 nephridium. 



An excretory 

 function has also 

 been assigned to 

 a single pair of 



organs called the brown funnels (Fig. 752, br. /.), also situated on 

 the dorsal aspect of the pharynx at its posterior end. Their wide 

 backwardly-directed ends open into the atrium ; their narrow 

 anterior ends probably communicate with the ccelome. There are 

 also groups of columnar excretory cells on the floor of the atrium. 



'Nervous System. The central nervous system is a rod-like 

 organ, the neuron or dorsal nerve-tube (Fig. 751, A, n. ; B, neu., 

 Figs. 752, 754), contained within and completely filling a median 

 longitudinal neural canal which lies immediately above the noto- 



VOL. II E* 



FIG. 755. Amphioxus lanceolatus. Nephridium of the left 

 side with part of the wall of the pharynx. (From Willey, 

 after Boveri.) 



