IV 



PHYLUM CCELENTERATA 



153 



formed of a single layer of cells (Fig. 112, A). The blastula 

 elongates, and the cells at one pole undergo division, the daughter- 

 cells passing into the cavity, which they gradually fill (B). At t 

 this stage the embryo is called a planula : it consists of an outer 

 layer of cylindrical cells the ectoderm which acquire cilia, and an 

 inner mass of polyhedral cells the endoderm. In some cases the 

 planula arises by a different process : a solid morula is formed, the 

 superficial cells of which become radially elongated and form 

 ectoderm, the central mass of cells giving rise to endoderm. By 

 means of its cilia the planula swims freely, and before long a 

 cavity appears in the middle of the solid mass of endoderm, the 

 cells of which then arrange themselves in a single layer around 

 the cavity or enteron (C, al). The planula then comes to rest, fixes 

 itself at one end to some suitable support, and becomes con- 



FlG. 112. Early development of Eucope. A, blastula-stage ; B, planula with solid endo- 

 derm ; C, planula with enteric cavity ; al, enteric cavity ; ep. ectoderm ; hy. endoderm. 

 (From Balfour's Embryology, after Kowalevsky.) 



verted into a simple polype or hydrula by the attached end 

 broadening into a disc and the opposite extremity forming a 

 manubrium and tentacles. The hydrula soon begins to send off 

 lateral buds, and so produces the branched colony. 



In Tubularia the oosperm develops, while still enclosed in 

 the sporosac, into a short hydrula, which, after leading a free 

 existence for a short time, fixes itself by its proximal end, buds, and 

 produces the colony. In Hydra development begins in the ovary, 

 and is complicated by the fact that the ectoderm of the morula 

 gives rise to a sort of protective shell : in this condition the 

 embryo is set free, and, after a period of rest, develops into the 

 adult form. 



