4o6 THE CRANIATES AND THE ACRANIATES. 



recognizable, at the head end, the process extending backward with the growth 

 of the body. When viewed in cross-section, the mesoderm and notochord ap- 

 pear, at first sight, to be in the act of arising from the endoderm. (Fig. 271, G.) 

 As a matter of fact, what is really taking place is the belated separation of the 

 mesoderm bands, notochord, and endoderm from one another, while all of them 

 owe their origin to terminal groups of proliferating cells, just as in the typical 

 craniate embryos. 



An enteron, or primitive gut, does not exist till after the products of telo- 

 blastic growth have separated, and the lateral bands of the endoderm have united 

 to form a closed tube. The mesoderm may separate from the lateral walls of the 

 mesentocoele as hollow vesicles, either during (Amphioxus), or before its division 

 into somites, or coelomic chambers (echinoderms). In either case the process is 

 not a primitive, but a secondary one. It is merely another way of attaining the 

 same conditions seen in the annelids and arachnids, and is probably the result of 

 a rapid development of yolkless eggs. Owing to the almost universal absence 

 of a considerable volume of yolk in the acraniates, there is no terminal concres- 

 cence like that in the craniates. 



Mouth. — A functional neurostoma and primitive stomodseum are formed 

 between the anterior ends of the nerve cords in the cirripeds, chaetognatha, echino- 

 derms, brachiopods, and polyzoa. In all the other sub-phyla, a vestigial, or trans- 

 itory neurostoma is formed as a median depression in the anterior end of the 

 medullary plate. It opens outward, in those forms in which the anterior end of 

 the medullary plate is not infolded to form a closed forebrain vesicle (enterop- 

 neusta, pterobranchia, phoronida, polyzoa, chaetognatha. Where the medullary 

 plate is infolded and closed, the neurostoma lies in the floor of the forebrain 

 vesicle (tunicates, Amphioxus). The infolding for the primitive mouth may be 

 of considerable depth, forming a true primitive stomodasum, opening perma- 

 nently into the mesenteron as in the case of the dorsal tubercle and subneural 

 gland of the tunicates; or a blind pocket may arise from the midgut, that grows 

 toward the primitive mouth, representing either the cut off remnant of the primi- 

 tive stomodseum, or that part of the midgut that formerly communicated with it. 

 (enteropneusta, pterobranchia, (and phoronida ?)). 



The haemostoma is a new formation arising independently of the old mouth, 

 from the anterior haemal surface of the body. 



The Naupula. — The larva of the acraniates, or the naupula, is a small, 

 usually free swimming, pelagic form resembling a cirriped nauplius. It may 

 undergo a part of its development in the brood pouches, or recesses, of the atrial 

 chamber (cirripeds, tunicates, echinoderms, brachiopods, and polyzoa). The 

 naupula differs from a trochosphere in that it represents a distinctly older phylo- 

 genetic stage, and undergoes a special kind of metamorphosis. It possesses a 

 longitudinal, circular fold, representing a larval mantle fold or carapace, that 

 usually has a ciliated margin. The larva comes to rest, neural side down, and 

 becomes permanently attached, usually by an adhesive disc, aided by rudimentary 



