ON THE DEVKLOPI^IENT OF SAGITTA. 371 



directly in connection with this migration, and in a distinctly 

 different way from that which gives rise to the anterior 

 septum between the head-cavities and the rest of the coelom. 

 The theoretical bearing of these points Avill be discussed more 

 fully later. 



The completion of the septum between the trunk and tail 

 brings to an end the period of development which may be 

 described as larval, for the animal has now essentially the 

 form and structure of an adult. It still differs from the 

 latter in many important respects, — for example, the ovaries 

 and testes of each side are still each represented by a single 

 cell, and there are no genital ducts. The alimentary canal is 

 without a lumen, except in the mouth region, and the anus 

 has not yet been formed. The abdominal ganglion is also 

 enormously larger than in the adult, relatively to the size of 

 the animal; there are no anterior fins, the parenchymatous 

 tissue in the neck region and behind the ganglion is very 

 conspicuous, and many minor differences still exist. The 

 changes, however, which transform the young of this stage 

 into the adult condition are very gradual, extending over 

 several weeks, if not more, and comparatively few could be 

 observed in the young Sagitta raised from the egg. 



During the first few days after hatching the alimentary 

 canal increases steadily in thickness, and its nuclei become 

 prominent; but the thicker portion ends abruptly opposite 

 the genital cells, so distinguishing the true gut from the 

 longitudinal tail septum, with which it is continuous. A 

 lumen begins to appear in the alimentary canal shortly after 

 the reappearance of the cuslom. As the ccelomic cavities 

 grow wider the alimentary canal becomes supported by a 

 dorsal and ventral longitudinal mesentery, and about the 

 seventh day its end can be seen bending down in the 

 mesentery to touch the ectoderm. On the eighth or ninth 

 day the anus is formed at the junction of ectoderm and 

 endoderm ; it is not exactly opposite the transverse septum, 

 but a short distance in front of it, and a space is left between 

 the two layers of splanchnic mesudurm, extending from the 



