292 Universihj of California Publications. [Zoology 



niately equidistant, may arise simultaneonsly jnst within the 

 circle of proximal tentacles. They are the beginnings of pedun- 

 cles which later bear medusae. They increase in number grad- 

 ually throughout the life of the polyp, by the addition of new 

 buds between those already formed. They never equal the 

 number of proximal tentacles. They are larger than the latter, 

 and include at the start a portion of the hydranth cavity. En 

 doderm and ectoderm are equally involved in the development, 

 which is initiated as a simple evagination. Medusae may arise 

 on the peduncles at an early age, one of them usually developing 

 bj' a transformation of the end of the peduncle, the others as 

 lateral buds. The peduncles themselves may branch several 

 times. The detailed development of the medusa will not be fol- 

 lowed. The reader is referred for it to Allman (71). 



VIII. — Summary and Conclusion. 



The more important facts in the post-embryonic period may 

 now be briefly summarized. 



The larva is never ciliated or free swimming. Its emergence 

 from the egg case is governed by two sets of factors. Internal 

 factors determine an area of accelerated ditferentiation which 

 marks the point at which the envelope is dissolved. External 

 factors such as contact and possibly gravity determine the kind 

 of structure (e. g. hydranth or holdfast) which will ultimately 

 appear in connection with the area of differentiation. That is, 

 they determine the polarity of the adult. 



The larva elongates as it emerges. Its free (oral) region 

 enlarges and its aboral region narrows. The latter adheres to 

 the substratum, on which it moves by means of amoeboid cells 

 in the ectoderm ; the capacity for locomotion is never lost. The 

 elongation of the body is accompanied by a gradual vacuolation 

 of the cells of the endoderm. 



The body is next divided into hydranth and stem regions by 

 an external constriction arising from a shortening of the colum- 

 nar ectoderm cells in its path, and a corresponding internal 

 ridgelike thickening of the endoderm arising from the elonga- 

 tion of the columnar cells in its path. The ridge is more con- 



