PROTOCHORDATA 



66 1 



municates on the left side (and in the Hemichorda often on both sides) 

 by a pore with the exterior. In the Hemichorda the three segments 

 retain their entity throughout life : the first does not divide into lateral 

 halves, the second (collar cavities) acquires a pair of pores to the ex- 

 terior, the third (trunk cavities) does not undergo transverse division. 

 In the Cephalochorda, the first divides into two halves, of which the 

 left, by the opening out of its pore, becomes Hatschek's pit in the 

 ectodermal depression known as the wheel organ, the second forms 

 the first mesoderm segment (mesoblastic somite) and some cavities 

 around the mouth, the third subdivides to form all the mesoderm 

 segments except the first. In the Vertebrata the coelom forms as a 

 split in a mass of mesoderm, though there are indications that the 

 mesoderm rudiment should be regarded as a solid pouch arising in 



coe. 3'. 



Fig. 462. A diagrammatic longitudinal section of an embryo of Amphioxus. 

 From Shipley and MacBride. al. alimentary canal; coe. i. anterior coelom 

 or head cavity; coe. 2. middle coelom (collar cavity), which becomes first 

 mesoblast segment ("mesoblastic somite"); coe. 3. hinder coelom (trunk 

 cavity); coe. 3'. mesoblast segment dividing off from coe. 3; n. neural canal; 

 ne. neurenteric canal; neii. neuropore. 



the same position as the hollow pouches of the Cephalochorda. The 

 head cavity is represented by the premandibular segment of the em- 

 bryo, a median structure with an opening to the exterior in the form 

 of a communication with the ectodermal invagination for the pituitary 

 body. Certain peculiarities of the mandibular segment indicate that 

 it is the homologue of the first mesoderm segment in the Cephalo- 

 chorda and so of the collar cavity. The remaining segments must re- 

 present those of the Cephalochorda and so the trunk cavity of the 

 Hemichorda. In the Tunicata the mesoderm arises as a solid mass 

 in the same position as the pouches of the Cephalochorda, but the 

 coelom, except for certain doubtful vestiges, is non-existent. 



(4) Except in the Hemichorda, the notochord, a skeletal rod which 

 arises from the endoderm of the median dorsal line of the gut, runs 

 the whole, or a considerable part, of the length of the body. In the 



