THE ECTOPROCTA. 



443 



embryonic tissues takes place. The degenerating cells are derived from two 

 principal sources, the hasmal blastoderm and the heemal musculature of the anterior 

 part of the thorax. They concentrate at a definite point on the haemal surface of 

 the head, where they are invaginated en masse into the yolk, and thence into the 

 enteron where they are absorbed. 



The haemal blastoderm cells are always invaginated into the yolk and ab- 

 sorbed. The degenerating lateral plate cells of the thorax certainly do not, as a 

 rule, undergo this fate, although it is difficult at times to distinguish between 

 them and those derived from the haemal blastoderm. The vast majority of them 

 either degenerate in situ, become free blood-corpuscle-like cells, or regenerate into 

 scattered muscle cells. See Chapter XIII, p. 230. 



The cephalic navel of arthropods is regarded as furnishing the initial condi- 

 tions that lead to the formation of a haemostoma. Pedicellina is the only case 



Fig. 302. — LarvK of polyzoa in the naupula stage, illustrating the degenerative, or pauperitic development 

 of the first generation. A, Pedicellina; B, Cyphonautes; C, Lepralia; D-E, Ctenostomidas; F, Bugula. (After 

 Barrois, Prouho, and Harmer, slightly modified. Semi-diagrammatic. 



known to me where the seat of degeneration appears to be in temporary communi- 

 cation with a definitely formed enteron. 



Conclusion. — The main features in the structure and development of the 

 entoprocta agree with those of primitive arthropods, and not with those of the 

 molluscs, or annelids, or with any other worm-like animals. In place of the 

 gastrula, blastopore, and trochosphere that are typical of annelids, we have the 

 teloccele, telopore, and naupula that are typical of the arthropods. 



The larva resembles that of cirripeds, in passing its early stages of develop- 

 ment within a brood pouch; in its subsequent mode of attachment and rotation; 

 in the mode of growth of its cephalic stalk and mantle, and in the histolytic 

 changes in the region of the cephalic navel; and in the location of the genital cells 

 and principal ganglia. 



The Ectoprocta. — The ectoprocta may be regarded as modified derivatives 

 of the more primitive entoprocta. They present an extraordinary diversity of 



