588 WILLIAM PATTEN. 



of the ring and containing near its centre a single nucleus 

 {PL XXXVIc, fig. 30). 



Just before the embryo hatches all the yolk-cells have 

 migrated to the periphery of the yolk, where they arrange 

 themselves to form the epithelial lining of the mesenteron. 

 About the same time the thin membranes at the blind ends of 

 the stomodaeum and proctodseum rupture, and their cavities 

 become continuous with that of the mesenteron. 



The remaining organs of the body which were already 

 formed during the preceding stages do not undergo any im- 

 portant changes during this period. 



The Larva. 



The larva, which resembles that of Mystacides figured by 

 Zaddach, immediately after its escape from the egg forms a 

 case of fine sand and small fragments of vegetable matter. 



In larvse several weeks old no gills were present unless the 

 vesicular appendages between the anal stylets can be considered 

 as such. When, however, the larva is about three sixteenths 

 of an inch long a pair of filamentous tracheal gills bud from 

 the lateral walls of each segment of the abdomen. 



Sections of very young larvae show the complete formation of 

 all the larval organs. The body cavity is broken up into irre- 

 gular spaces bounded by thin membranes, on the walls of which 

 there may be seen a number of large cells whose protoplasmic 

 filaments extend across the cavity, and either unite with the 

 opposite wall or with similar filaments from other cells 

 (PI. XXXVIc, fig. 29). These cells are the same that we have 

 already observed in the preceding stages attached to the body 

 wall, and which had migrated into the body cavity from the 

 yolk-sac. 



The salivary glands have already assumed the peculiar and 

 characteristic appearance that they have in the adult. A cross 

 section through one of them (PI. XXXVIc, figs. 25 and 26) 

 shows in the centre an exceedingly small duct surrounded by a 

 zone of radiating filaments; outside the latter, and constituting 



