290 



NA TURA L HIS TOR Y. 



THE TUNICATES AND THE LANCELET. 



The union of the two groups represented in our heading is indefensible 

 on scientific grounds ; they are widely different in structure, as we shall 

 see ; but they are here considered together since they agree in and differ 

 from all the rest in lacking the jointed backbone or series of vertebrae 

 common to all other vertebrates. 



The tunicates are marine forms, which are but little known to any but 

 the man of science, although some of them attain considerable size. In 



the older works on 

 zoology they were 

 classed with the mol- 

 luscs, with which,, 

 however, they really 

 have not the slight- 

 est affinity, although 

 they exhibit many 

 analogies. Their true 

 relationships were not 

 suspected until their 

 d evelopment was 

 studied, and then it 

 was seen that their 

 young were strikingly 

 like tadpoles in ap- 

 pearance. And far- 

 ther, they had a very 

 similiar internal struc- 

 ture. Both breathed 

 by gills, and the water 

 to aerate the blood 

 was drawn in the 

 mouth, and then was 

 forced out through 

 slits on the sides of 

 the throat, exactly as 

 occurs in the adult 



Fig. 287. — A simple tunicate (Phallusia). 



fish ; farther, both have a cartilaginous rod (the notochord) in a position 

 Avhich exactly corresponds to the centre of the vertebral column of the 

 frog, but which entirely disappears in the adult tunicate. Farther than 



