METAMORPHOSIS. 



435 



An outgrowth of the enteron extends toward, and then unites with it, to form the 

 hsemostoma; meantime the surrounding haemal surface is greatly shortened by the 

 drawing together of the anterior and posterior ends, the progress of the contraction, 

 which may be compared with the reversed curvature commonly seen in arthropod 

 embryos during the formation of the "dorsal organ," being indicated not only by 

 the general shape of the larva, but in a specially striking manner by the changes 

 in the form of the ciliated band, B, C and D. 



Metamorphosis. — During the metamorphosis the ciliated bands disappear, 

 and the larva rapidly takes on the form of the adult. (Fig. 296, E.) The preoral 

 region elongates and forms the proboscis, or procephalon, the middle section 

 forms the collar, or thorax, and the posterior one, the abdomen, or tail. 



Cephalic Ccecum. — Meantime a median outgrowth, or cephalic csecum, 

 arises from the anterior end of the enteron, that marks the beginning of the so- 

 called "notochord" (Fig. 296, E.) But neither in structure, nor origin, nor in its 

 anatomical relations, has it any resemblance to a notochord, for we have seen in 



Pig. 297. — Diagrams illustrating the structure of Balanoglossus. A, Sagittal section through the procephalon 

 (proboscis) and mesocephalon (collar, thorax); B, transverse section of the mesocephalon ; C, transverse section of 

 the abdominal, or branchial region. 



Chapter XIX, that the notochord is a modification of the middle cord, that it arises 

 from the ectoderm, and that it is never connected with or forms a part of a func- 

 tional alimentary canal. It is therefore obvious that an organ cannot be re- 

 garded as a notochord merely because it is a diverticulum of the gut. In fact, such 

 an origin or connection may be accepted as conclusive evidence that the organ in 

 question is not a notochord. 



The cephalic csecum of Balanoglossus, in its minute structure and location 

 is not essentially different from the caecal outgrowths of the foregut which 

 are so common in arthropods, as for example in cirripeds (Figs. 275 and 277), 

 entomostraca (Fig. 282, A), phyllopods (Fig. 273), cladocera (Fig. 10), and 

 arachnids (Fig. 43). 



The cephalic csecum of the enteropneusta probably represents the remnants 

 of that part of the foregut that originally opened into the primitive stomodsecum 

 and neurostoma. 



The Late Larval and the Adult Stages. — In the later stages the principal events 

 are: The further differentiation of the main subdivisions of the body, that is, of the 

 procephalic, thoracic, branchial, and caudal regions; and the development of 

 the gill slits, lateral folds, endocranium, excretory ducts, and nervous system. 



