Xrtt 



MYLUM [CHORDATA 



19 



iffh. 



description that follows, therefore, the word vessel is not used in 

 its strict sense. At each end of the heart is given off a large 

 " vessel." That given off ventrally, the branchio-cardiac vessel 

 (br. car.), runs along the middle of the ventral side of the pharynx 

 below (externally to) the endostyle, and gives off a number of 

 branches which extend along the bars between the rows of stigmata, 

 and give off smaller branches passing between the stigmata of 

 each row. The vessel given off from the dorsal end of the heart 

 the cardio-visceral (card, vise.) breaks up into branches which 

 ramify over the surface of the alimentary canal and other organs. 

 This system of visceral vessels or lacunae opens into a large sinus, 

 the viscero-branchial vessel, which runs 

 along the middle of the dorsal wall of 

 the pharynx externally to the dorsal 

 lamina, and communicates with the 

 dorsal ends of the series of transverse 

 branchial vessels. In addition to these 

 principal vessels there are numerous 

 lacunae extending everywhere through- 

 out the body, and a number of branches, 

 given off both from the branchio- 

 cardiac and cardio-visceral vessels, 

 ramify, as already stated, in the sub- 

 stance of the test. The direction of 

 the circulation through the main vessels 

 differs according to the direction of the 

 heart's contractions. When the heart 

 contracts in a dorso-ventral direction, 

 the blood flows through the branchio- 

 cardiac trunk to the ventral wall of 



the pharynx, and through the trans- FIG. 729.-Asci<ua. Dorsal tubercle, 

 verse vessels, after undergoing oxygena- 

 tion in the finer branches between the 

 stigmata, reaches the viscero-branchial 

 vessel, by which it is carried to the 

 system of visceral lacunae, and from 

 these back to the heart by the cardio-visceral vessel. When the 

 contractions take the opposite direction, the course of this main 

 current of the blood is reversed. 



The nervous system is of an extremely simple character. There 

 is a single nerve-ganglion (Figs. 725 and 727, ne. gn., 729, gn., and 

 730, n.g.) which lies between the oral and atrial apertures, embedded 

 in the mantle. This is elongated in the dorso-ventral direction, 

 and gives off at each end nerves which pass to the various parts 

 of the body. 



Lying on the ventral side of the nerve-ganglion is a body the 

 neural gland (Figs. 725, 727, hyp. ; Fig. 729, gld., and Fig. 730, 



c 2 



nerve-ganglion, and associated 

 parts as seen from below, del. duct 

 of neural gland ; dors. lam. dorsal 

 lamina ; gld. neural gland ; gn. 

 ganglion ; hyp. dorsal tubercle ; 

 nv., nv. nerves ; periph. peri- 

 pharyngeal band. (After Julin.) 



