The Tunicates, or Ascidians 469 



enlarged anterior end of the digestive tract ; and as the holes, 

 or stigmata, as they are technically called, are perforations of 

 the wall for the passage of water for purposes of respiration, 

 they are both morphologically and physiologically comparable 

 with the gill openings of fishes." There can be no doubt, there- 

 fore, that the pharyngeal sac of Ascidians is homologous with 

 the pharynx of fishes. 



Surrounding the mouth, or branchial orifice, just at its 

 entrance into the branchial chamber is a circle of tentacles. 

 These are simple in some genera, but elaborately branched 

 in others. 



In close connection with the cerebral ganglion, which is 

 situated between the two siphons, there is a large gland with a 

 short trumpet-shaped duct opening into the branchial sac a 

 "ittle distance behind the mouth. The orifice of the duct is 

 just within a ring consisting of a ciliated groove that extends 

 around the mouth outside the circle of branchial tentacles. 

 On the opposite side of the mouth from the gland the ciliated 

 groove joins another groove which is both ciliated and glandular, 

 and which runs backward along the upper floor of the pharyn- 

 geal sac to its posterior extremity. This organ, called the 

 endostyle, is concerned in the transportation of the animal's 

 food through the pharyngeal sac to the opening of the oesopha- 

 gus. Comparative embryology makes it almost certain that 

 the subneural gland with its duct, described above, is homologous 

 with the hypophesis cerebri of true vertebrates, and that the 

 endostyle is homologous with the thyroid glands of vertebrates. 



The water after passing through the branchial network is 

 received into narrow passages and conducted to a larger cavity — 

 the cloacal or atrial chamber. The general relations can be 

 seen from our diagram, illustrating a vertical and horizontal 

 section. From the atrial chamber the water flows out into the 

 external world. 



Now we can readily see how in the older works naturalists 

 were misled as to the affinities of the Tunicates. They re- 

 garded the tunic as the equivalent of the mantle of the mol- 

 lusks, while the mcurrent and excurrent openings corresponded 

 to the siphons. In one genus, Rhodosouia, the resemblance 

 was even stronger, for there the tunic is m two parts, united 



