2 8o COMPARATIVE ANATOMY 



lation i.e. the folding of one half of a hollow blastula into 

 the other half so as to produce a two-layered sac or gastrula 

 opening to the exterior by a single aperture called the 

 blastopore. But in Clytia there is no blastopore. The result 

 of immigration is a solid embryo, having an external layer 

 of columnar cells forming an ectoderm and an internal solid 

 mass representing an endoderm. This embryo elongates 

 and becomes pear-shaped and the ectoderm cells acquire a 

 covering of cilia. In this stage it is known as a planuia, a 

 larval form very characteristic of the majority of Coelenterata 

 but not found in Hydra. The planuia swims about for some 

 time by means of its cilia, and during its free existence a split 

 appears in the central mass of endoderm cells, forming the 

 first rudiment of the gastro-vascular cavity. After a time it 

 settles down by its broader end on some convenient sub- 

 marine object, throws off its coat of cilia and begins to grow 

 into a hydriform person, the founder of a new colony. The 

 surface of attachment spreads out to form a disc clinging to 

 the weed or stone to which it is fastened, and the edges of 

 the disc soon become divided up into lobes which grow out 

 and form the branches of the root-like hydrorhiza. The 

 embryo elongates, its distal portion swells into a club shape, 

 it loses its coat of cilia and the gastro-vascular cavity enlarges 

 whilst the endoderm cells take on the characters of the adult 

 endodermic epithelium. A mouth is soon formed at the 

 distal extremity, a circlet of tentacles grows out round the 

 mouth, and a chitinous tube, the perisarc, is secreted by 

 the ectoderm. The first hydra person is now complete. It 

 soon gives off a bud, and by continued repetition of the 

 budding process a new colony is formed ready to go through 

 the same life cycle as that just described. 



A life-history as complicated as that of Obelia presents 

 many problems for consideration. In the foregoing descrip- 

 tion, by calling the different members of the colony by the 

 name of " persons" it has been tacitly assumed that they are 

 really individuals, which have only become secondarily united 

 to form a composite organism. And indeed one kind of 

 person, the medusa, displays a high degree of individuality 

 inasmuch as it leads a free and separate existence. But why 

 are the other members of the stock or colony to be regarded 

 as individuals united together rather than as analogues of the 



