44 2 THE POLYZOA. 



ings separated by a deep infolding, one next the anterior wall of the stomodceum, 

 the other in front of the rectum, t.g. The two ganglia may be regarded as 

 representing the postoral and the anal group of ganglia of the nauplius. 

 (Fig. 272.) 



On either side of the stomocteum is an excretory duct (Fig. 301, H. I. c.d.), 

 that corresponds approximately with the cephalic excretory duct of the nauplius. 

 The genital cells and genital ducts appear to arise from the deep infolding between 

 the postoral and postanal ganglia, in a position, therefore, that corresponds to 

 the location of the same organs in the nauplius, in Cephalodiscus, and in the adult 

 stages of many arachnids. 



The early development of the relatively large brain and nerve cord definitely 

 locates the primitive neural surface, and gives us the necessary data for the proper 

 orientation of the larva. The latter comes to rest in the typical naupula fashion, 

 neural side down. (Fig. 300, E.) A large cephalic outgrowth is then formed, and 

 as it elongates, it straightens out and lifts up the body, which meantime turns a 

 half somersault, bringing its neural surface uppermost. The edges of the pleural 

 folds, about the time of attachment, unite to form a closed vestibule, or atrial 

 chamber, at. r, within which the appendages make their appearance, F. During 

 the revolution of the larva, they elongate, rupture the vestibule, and finally pro- 

 trude from it in the same manner as the legs of a cirriped, or the arms of a young 

 crinoid. (Figs. 274, 295.) 



During the early stages of revolution, the brain and apical disc separate from 

 the ectoderm and break up into a mass of loose cells that nearly fill the cavity of 

 the cephalic stalk. (Fig. 301, F.) They appear to increase in numbers and to 

 receive accessions from other sources. The anterior wall of the enteron, or vesti- 

 bule according to Harmer, then breaks down, and its walls, surrounding an ill 

 defined space, merge with the degenerating cells in the stalk. Later some of the 

 cells undergo still further degeneration, whether by being bodily enclosed in the 

 cavity of the enteron as seems probable, or not, does not appear. In either case 

 the rupture in the walls of the enteron closes over and the amorphous mass of 

 degenerating cells decreases in volume. Some cells appear to persist in the 

 stalk as star-shaped, connective tissue cells. 



The point that appears to be clearly established is the disappearance of the 

 forebrain and apical disc by a process of histolysis, that seems also to affect other 

 adjacent tissues, including the anterior wall of the enteron. The fate of the free 

 cells thus produced is not clear. It is obvious that the whole process, even in the 

 way it affects the forebrain and apical plate, is very similar to those which occur 

 on the haemal surface of the thorax, and in the cephalic navel, or dorsal organ, of 

 cirripeds and parasitic copepods, and it occurs in a corresponding region, that is, 

 on the anterior haemal surface of the head, at the point where attachment takes 

 place. (Figs. 274, 282, 283.) 



It will be recalled that the cephalic navel of arthropods is an area on the 

 anterior kemal surface of the cephalothorax, where extensive degeneration of 



