188 W. K. BROOKS. 



the fishes show also that the process of degeneration has reached its 

 extreme in the tunicates. The cyclostomes are held to owe their 

 degeneracy to parasitism, and the most important element in the 

 more advanced process of degeneration is that the ascidians no 

 longer fasten themselves to fishes nor make use of their bodies as 

 food, but that they fasten themselves to stones, to ships, or to the 

 bodies of other animals which do not serve as food, such as the 

 shells of crabs or the tubes of annelids. 



The mouth (p. 57) which in the cyclostomes serves both as an 

 organ for attachment to the skin of fishes, and also as a sucker for 

 extracting their blood, has become converted in the ascidians into 

 an organ for attachment ; and these animals have thus lost their 

 old mouth, which was homologous 'with that of the true vertebrates, 

 and have acquired a new one which is homologous with the verte- 

 brate nasal chamber. 



The process, Dohrn says, must be represented as follows : The 

 fishes take in the water for respiration through the mouth, but as 

 this is used by the parasitic cyclostomes as a sucker, they have 

 acquired another arrangement, and the water is not only discharged 

 through the gill-slits, but i^ also inhaled through them, and, in 

 the myxenoids, through the nasal passage also, which has in the 

 tunicates become the functional mouth. The vertebrate mouth has 

 lost its old function in the cyclostome-like ancestors of the tunicates, 

 as these have gradually lost their parasitic habit, and have estab- 

 lished themselves on lifeless bodies ; but the original lips have 

 remained, and they are to be recognized in the so-called sucking 

 knobs of the ascidian larva, while the teeth of the cyclostomes are 

 supposed to be represented by " bristle-carrying end knobs " upon 

 the suckers. 



The " so-called larva " of the ascidians is represented in almost 

 every feature of its organization by the adult, sexually mature, 

 appendicularia. No better example of the correspondence between 

 an adult animal and an ontogenetic stage in the history of another 

 can be desired, and we may feel confident that, whatever the 

 phylogenetic history of appendicularia has been, that of the ascidian 

 larva has been the same. 



Nearly all of the students who have devoted themselves to the 

 study of the tunicates agree in regarding appendicularia as a per- 



