584 THE INVERTEBRATA 



on the left side (and in the Enteropneusta often on both sides) by a 

 pore with the exterior. In the Enteropneusta the three segments retain 

 their entity throughout hfe : the first does not divide into lateral halves, 

 the second (collar cavities) acquires a pair of pores to the exterior, 

 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 



coe. 3'. 



Fig- 437- A diagrammatic longitudinal section of an embryo of A?nphioxus. 

 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 ; w. neural canal; 

 ne. neurenteric canal; neu. neuropore. 



arising in the same position as the hollow pouches of the Cephalo- 

 chorda. The head cavity is represented by the premandibular segment 

 of the embryo, 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 

 Cephalochorda and so of the collar cavity. The remaining segments 

 must represent those of the Cephalochorda and so the trunk cavity 

 of the Enteropneusta. 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 Enteropneusta, 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 



