STRUCTURE OF THE VERTEBRATES 13 



Urochordata. The Urochordates, or Tunicates, are a large 

 group showing intimate connections with each other, and dis- 

 tinct relationship to the Cephalochordates. The larger forms 

 are familiar objects in shallow water along the sea coast, and 

 are there known as "Sea Squirts". Until their embryology was 

 studied the affinities of these animals were unknown, due to the 

 degeneration which they undergo in metamorphosis from larva 

 to adult. 



The larva of a typical tunicate has clearly defined chordate 

 characteristics. The mouth opens into a relatively large pharynx 

 with a few gill slits. Within the pharynx there is a median, ven- 

 tral groove, the endostyle, which collects food particles and 

 passes them into the short intestine. The intestine becomes 

 spiralled early in development. The nerve cord shows marked 

 advances over the hemichordate condition. The anterior ganglion 

 is well developed and often called a brain. Connected with it is 

 a light-sensitive organ, the so-called eye, and an otocyst which 

 functions as a balancing organ. The notochord is also well 

 developed, though it extends no farther anteriorly than the 

 posterior end of the pharynx. 



The larva leads a free-living existence for a short time and 

 then undergoes rapid metamorphosis into the adult form. At 

 this time it settles to the bottom and becomes attached to a 

 stone, wharf pile, or other object. The nerve cord rapidly degen- 

 erates; the pharynx increases in size with a multiplication of 

 gill slits; and the tail, with the entire notochord, is absorbed, the 

 animal feeding on its own tissues during the metamorphosing 

 process. Synchronously with the degenerative changes a tunic, 

 or cover, is secreted, surrounding the animal as a lifeless mantle 

 of animal cellulose. This bears the same relationship to the 

 animal as does the shell to a snail. The tunic has two openings, 

 one an incurrent pore, the other excurrent. The animal is then 

 a completely stationary organism carrying on nutritive and 

 reproductive processes. 



The tunicates, with the exception of the hemichordates, are 

 perhaps the most ancient line of the phylum. The larvae show 

 their group relationships, but during their evolution the adults 

 have become specialized in a degenerative manner. 



