CHORD AT A 439 



the genital ducts run for some distance along the dorsal 

 middle line, and with ■ these runs the dorsal blood-vessel. 

 The walls of the branchial sac are also connected with the 

 mantle at irregular intervals by blood-vessels, which pass from 

 the bars to supply the substance of the mantle. The branchial 

 bars run at right angles to one another, and leave, between 

 them small ciliated slits or stigmata, through which the water 

 taken in at the mouth escapes into the peribranchial chamber, 

 the current being maintained by the cilia. The transverse 

 bars contain blood-vessels communicating with the dorsal 

 and ventral blood-vessels. Connecting two adjacent transverse 

 bars are a number of small longitudinal bars, which divide the 

 space up into the stigmata. There are two kinds of transverse 

 bars, the primary and the secondary, the latter being the 

 smaller. In addition to the small longitudinal bars, there are 

 also some large ones situated in a different plane from the 

 others, farther inside the branchial sac; these are connected 

 by a short vessel with the transverse bars at their points of 

 intersection (Pig. 256), and at the same spot they give off 

 little finger-shaped processes, which hang into the lumen of 

 the sac. 



A cUiated groove, known as the hypopJiaryngeal groove or 

 endostyle, runs along the ventral line of the branchial sac 

 (Fig. 254). The lips of the groove project into the cavity 

 of the sac, and it is lined by cells which are partly glandular 

 and partly ciliated. At the posterior end it is continued into 

 a cul-de-sac, which has a slight spiral twist ; anteriorly its lips 

 are continuous with the peripharyngeal bands, two ciliated 

 ridges which encircle the anterior end of the branchial sac 



7. Stomacli. 16. Heart. 



8. Intestine showing typhlosole, part 17. Ovary. 



of it removed to show subjacent 18. Pore of vas deferens. The openings 

 structures. of the oviduct and the vas deferens 



9. Eectum. are shown enlarged to the right. 



10. Anns. 19. Testicular follicles on intestine ; the 



11. Atrial aperture. greater part of them have been 



12. Inner surface of mantle, showing removed with the intestine. 



longitudinal and transverse muscle 20. Oviduct. 



fibres. 21. Septum shutting off that part of 



13. Dorsal tubercle. the body -cavity which contains 



14. Subneural gland and brain. the heart, stomach, and generative 



15. Cut edge of branchial sac. organs. 



