504 



UROCHORDA 



is usually complex, aud forms the so-called dorsal tubercle, which 

 is very distinct on the wall of the pharynx, and of considerable 

 systematic importance. It lies at the point where the two halves of 

 the ciliated groove, or peripharyngeal band, already described, converge 



Pk.^ 



Fig. 286. —Diagram of Ascidian. — After Herdman. 



The arrows indicate tlie two openings ; the dark border the test. 

 Ph., Pharynx, with gill-slits ; G., reproductive organs ; H., 

 heart, with blood vessels ; G.D., genital ducts ; /?., rectum, 

 ending in cloacal chamber. Surrounding the pharynx the 

 peribranchial cavity is shown. 



dorsally to form the dorsal lamina. In Ascidia the sub-neural organ 

 is ventral to the brain, and partly glandular in character, and so it 

 is in many ; in some cases, however, it is dorsal in position, and its 

 glandular portion is reduced to nil. It is probable that the sub- 

 neural gland and its duct correspond to the olfactory pit of Amphioxus, 

 and perhaps to the hypophysis of Vertebrates. 



