Phylum Chordata 561 



dorsal tubular nervous system is one way in which the chordates are so 

 clearly differentiated from the higher invertebrates. Most of these latter 

 have a solid nerve chord which is ventral in position. 



Subphyla of the Chordata. — This phylum is divided into three 

 subphyla on the basis of the degree of notochordal development and 

 other accessory structures. 



Subphylum I. Urochordata. Adults saclike, may be either sessile or pe- 

 lagic. The larvae closely resemble the tadpole of the amphibians, and the noto- 

 chord is present only in the tail. Examples : tunicates, Molgula and Ciona. 



Subphylum II. Cephalochordata. Small fishlike animals, the adults with 

 a complete notochord. Example: Branchiostoma, commonly called amphioxus. 



Subphylum III. Verlebrata. With vertebrae variously developed, with an 

 internal jointed skeleton and other chordate characteristics variously modified. 

 Examples : fish, amphibians, reptiles, birds, and mammals. 



The first two subphyla are known as the protochordates ; both are en- 

 tirely marine in their distribution. The vertebrates are inhabitants of 

 all possible places. 



THE SUBPHYLUM UROCHORDATA 



An adult tunicate bears but little resemblance to the commonly ac- 

 cepted idea of what constitutes a chordate. It is a saclike animal pro- 

 vided with the incurrent and excurrent siphons and a set of highly modi- 

 fied organs. The outer test or tunic is composed of a cellulose-like ma- 

 terial, tunicin. It is through the larval form that the true relationships 

 of this peculiar protochordate are shown. Superficially the larva re- 

 sembles a frog tadpole and possesses a well-developed notochord in the 

 tail. The members of one class, the Larvacea, remain permanently as 

 larvae. 



The Classes of the Subphylum. — This subphylum is divided into 

 three classes which are as follows : 



Class I. Larvacea. Small transparent fornis abundant as part of the sur- 

 face-floating organisms, the plankton ; permanently in the tadpole stage. Herma- 

 phroditic, functioning first as males, then as females. Example : Appendicularia. 



Class II. Ascidiacea. Adult animal sessile, may occur as colonial or single 

 forms, with tadpolelike larva. Example : tunicates, Molgula, Ciona. 



Class III. Thaliacea. Transparent animals which usually dwell near the 

 surface in the open ocean. At one stage, several attach together to form circles 

 or long bands. Commonly there is an alternation of sexual and asexual repro- 

 duction. Example : chain tunicates, Salpa. 



