The Protochordata 461 



there is no brain, the notochord is confined to the tail and is 

 usually present only in the larval stage of the animal when it 

 has the form of a tadpole. In later life the animal usually 

 becomes quiescent, attached to some hard object, fixed or float- 

 ing. It loses its form and has the appearance of a hollow, 

 leathery sac, the body organs being developed in a tough tunic. 

 There are numerous families of Tunicates and the species are 

 found in nearly all seas. They suggest no resemblance to 

 fishes and look like tough clams without shells. The internal 

 cavity being usually filled with water it is squirted out through 

 the two apertures when the animal is handled. The class 

 Enteropneusta (Adelochorda, or Hemichordata), includes the rather 

 rare worm-like forms related to Balanoglossus. Bateson has 

 shown that these animals possess a notochord which is devel- 

 oped in the anterior part of the body. They have no fins 

 and before the mouth is a long proboscis. Gill-slits are found 

 in the larval tunicate. In Balanoglossus these persist through 

 life as in the fishes. 



The remaining chordate forms constitute the vertebrates 

 proper, not worm -like nor mollusk-Hke, the notochord not 

 disappearing with age, except as it gives way, by specialized 

 segmentation to the complex structures of the vertebral column. 

 These vertebrates, which are permanently aquatic, are known 

 in a popular sense as fishes. The fish, in the broad sense,, is 

 a backboned animal which retains the homologue of the back- 

 bone throughout life, which does not develop jointed limbs, 

 its locomotive members, if present, being developed as fins, 

 and which breathes through life the air contained in water 

 by means of gills. This definition excludes the Tunicates and 

 Enteropneusta on the one hand and the Amphibia or Batrachia 

 with the reptiles, birds, and mammals on the other. The 

 Amphibia are much more closely related to certain fishes than 

 the classes of fishes are to each other. Still for purposes of 

 systematic study, the frogs and salamanders are left out of 

 the domain of ichthyology, while the Tunicata and the Enterop- 

 neusta might well be included in it. 



The known branchiferous or gill-bearing chordates living 

 and extinct may be first divided into eight classes the Enterop- 

 neusta, the Tunicata, the Leptocardii, or lancelets, the Cyclostomi^ 



