56 



INVERTEBRATA 



CHAP. 



the opposite or oral end is a spot where ectoderm and endoderm thin out, 

 and there eventually the mouth will be formed by a perforation of both 

 layers. The disc-shaped embryo grows rapidly in the direction of its 



FIG. 30. Section of embryo of Tubularia mesembryantliemum showing the formation 

 of the aboral tentacles. (After Brauer.) 



ali.t, aboral tentacle ; coe, coelenteron ; o, spot where the mouth will be formed. 



principal axis and becomes cylindrical, and finally develops round its 

 oral end a series of small protuberances which are the rudiments of 

 the oral tentacles of the adult. By this time the mouth has been 

 formed and the aboral tentacles have become long and slender 

 (Fig. 31). 



A 



D 



Fi. 31. Five views of external features of different stages in the development of the 

 embryo of Tulularia indivisa. (After Allman. ) 



A, oval embryo. B, aboral concavity appears. C, rudiments of aboral tentacles. D, tentacles long ; 

 embryo becomes cylindrical. E, oral tentacles formed, (ib.t, aboral tentacles ; o.t, oral tentacles. 



The embryo is now called an Actinula, and is ready to leave the 

 bell of the gonophore. It escapes from its nursery, and creeps about 

 on the bottom of the sea with its mouth turned downwards, but 

 finally it attaches itself by the aboral end. It then rapidly grows 

 in height, and from its sides daughter persons are budded, so that 

 it forms an upright shoot in the adult colony. From its base arise 



