596 INVERTEBRATE MORPHOLOGY. 



CHAPTEE XVII. 



TYPE PROTOCHORDATA. 



THE type Protochordata contains a number of forms which 

 present certain features of similarity to the Chordata (Verte- 

 brata), one member of the type, Amphioxus being frequently 

 considered as belonging to that group, which is to be regarded 

 as the most highly differentiated of all the types composing 

 the Animal Kingdom. 



The various groups of the Protochordata differ greatly in 

 general appearance, but certain structural features of great 

 morphological importance are common to all of them. These 

 may be briefly stated as (1) a notochord, consisting of a more 

 or less well-developed rod, arising from the mid-dorsal line 

 of the digestive tract and either extending the entire length of 

 the body, or else limited to its anterior or its posterior part, 

 or even present only during larval life, as in the majority of 

 the Tunicata ; (2) branchial slits which place the cavity of the 

 pharynx in communication with the exterior and serve as 

 respiratory organs ; (3) a central nervous system, situated in the 

 mid-dorsal line of the body, and arising in some forms as ail 

 ectodermal iuvagination. 



Metamerism is but feebly indicated in the majority of 

 cases, some forms possessing only three mesodermal somites, 

 while others, such as some of the Tunicata, show traces of 

 it only in the posterior region of the body, Amphioxus being 

 the only form in which it is at all well marked. Limbs 

 do not occur in any members of the group, nor are there any 

 special jaws or organs of mastication. All the members of 

 the group are marine, and the various classes possess a wide 

 geographical distribution. 



I. CLASS HEMICHORDA. 



The members of this class are characterized by the noto- 

 chord being a comparatively small diverticulum of the an- 



