30 PHYLUM CEPHALOCHORDA. 



vein, the hepatic vein, which hke the sub-intestinal vein consists 

 of a plexus of vessels (Fig. 10). These commence in front and 

 behind unite to form a single vein which is continued into the 

 hind end of the sub-pharjmgeal vessel. The hepatic vessels are 

 said to communicate at the front end of the caecum with the 

 dorsopharyngeal coelom, but this statement must be accepted 

 with caution. There is undoubtedly a connection between the 

 anterior end of the caecum and the lateral wall of the atrium. 

 The blood is colourless ; it contains amoeboid cells, and accord- 

 ing to some observers a few red oval corpuscles. 



In the larva the hind end of the subintestinal vein is continued directly 

 into the sub-pharyngeal vessel. The direction of the flow of blood is 

 forward in the subintestinal vein and subpharyngeal vessels, both of which 

 are, according to J. Miiller, contractile. It follows from this that 

 the flow must be dorsalwards in the pharyngeal bars, backwards in the 

 dorsal aorta and ventralwards in the peri-intestinal vessels. There is a 

 considerable vascular development in the lateral walls of the atrium and 

 a longitudinal vessel runs along the line of the gonads, but how these 

 and the body- wall vessels generally are related to the main trunks described 

 above is not known. 



The other spaces of the body may be classed as lymph-spaces. They 

 are lined by an epithelium and contain a coagulable fluid. Their origin 

 and relations are not certainly known. Some of them, e.g. the lymph 

 canals in the fins and certain spaces within the myotomes are said to be 

 coelomic and derived from the mesoblastic somites of the embryo. Others 

 may possibly be purely vascular ; e.g. the large canal found in each meta- 

 pleure — the metapleural lymph canals. 



Generative organs. The sexes are separate. There are no 

 external sexual differences. Generative ducts are absent, 

 and the generative organs, which are segmented in cor- 

 respondence with the myotomes and are placed in the 

 lateral wall of the atrium at the ventral ends of the myo- 

 tomes, dehisce their products into the atrial cavity by 

 rupture of their walls. From the atrium the generative 

 products pass to the exterior usually through the atrial 

 pore, but in some cases, according to the observations of 

 Kowalevsky and Hatschek on the living, and of Marshall on the 

 preserved animal, they occasionally pass from the atrium through 

 the gill-slits into the pharynx and are spawned by the mouth. 

 Spawning takes place at sundown only and fertilization is effected 

 either in the sea or in the atrium. 



In a fully-developed Amphioxus lanceolatus the gonads are 

 somewhat cubical bodies, twenty-six in number on each side. 



