THE ENTOPROCTA. 



441 



structure, and they are disguised by the loss of important larval organs, the result of 

 an extensive degenerative histolysis; but in the structure and development of the 

 larvae and in their attachment and metamorphosis, indications of arthropod affini- 

 ties may be recognized. 



The entoprocta have undergone the least modification, and one of them, 

 Pedicellina, will best serve to illustrate the salient characters of the group. 



The young, as in so many other acraniates, undergo the earlier stages of 

 development within the brood pouches of the atrial or vestibular chamber. There 

 is a total and nearly equal cleavage, and the flattened, hollow blastula is infolded 



Fig. 301. — Development of an endoproctous polyzoan (Pedicellina). A—D, Formation of the telocoele, and the 

 early larval, or naupula, stages; E-G, mode of attachment, and metamorphosis; H—I, early and late larval stage, 

 seen from the neural surface. The most significant features are the absence of a true gastrula; the conspicuous 

 mantle, its formation of a vestibular, or atrial, chamber that encloses the appendages and the whole neural surface 

 of the body; the attachment of the larva by means of a large cephalic stalk; the rotation; and the degeneration 

 of the prosencephalon. Semi-diagrammatic. (In part after Hatscheck, Harmer, and Barrois.) 



at the caudal end to form a telocoele. (Fig. 301, A.) The telopore closes and an 

 independent ectodermic infolding at the head end unites with the enteron, forming 

 the primitive stomodasum and neurostoma, n.st. A typical gastrula stage, and a 

 blastopore, therefore, does not occur. The larva is a naupula, not a trochosphere, 

 as shown by the prominent labrum, by the longitudinal ciliated band represent- 

 ing the thickened margin of a branchial fold, and by the teloccelic method of 

 forming the germ layers. The enteron is U-shaped, with the concave side turned 

 toward the neural surface. There is a conspicuous apical disc, d, and a large 

 preoral ganglion, or forebrain, that is probably united by circumoral commissures 

 with the rudiments of a ventral nerve cord. The latter appears as two thicken- 



