IV COELENTEEATA 99 



Coelenterata. Now the planula appears under two forms: in 

 Scyphozoa and in some Actinozoa as a hollow two-layered vesicle 

 with a terminal opening, and in Hydrozoa and many Actinozoa 

 as a solid mass of endoderm surrounded by a skin of ectoderm. 



Which of these two is the more primitive and which the derived 

 form ? In answer to this question we say at once, the first of these is 

 the more primitive, because it alone exhibits a structure which is 

 a physiological possibility for a self-supporting animal. A solid 

 internal mass of cells would be quite functionless in an animal that 

 had to get its own living. 



The stage of the planula in development is preceded by the stage 

 of the hollow blastula, in all oases which have been thoroughly ex- 

 amined. The hollow blastula is changed into the planula either (a) 

 by intucking or invagination of one end, (6) by active proliferation of 

 ceUs proceeding from one end and filling up the interior, or (c) by 

 proliferation of cells from the whole internal surface. "We have 

 already pointed out that, whereas (c) is described only in cases of 

 eggs developing rapidly inside the bell of a vestigial medusa, or 

 with a shortened development in which the hydroid stage is almost 

 eliminated and the egg develops directly into a medusa, and that 

 the assertion of its occurrence even in these cases may be based on 

 a mistake in the observations ; on the other hand (a) and (6) occur 

 in eggs with a long larval development. 



We may take it therefore that these latter methods of endoderm 

 formation represent the least modified form of development, and that 

 the ancestral blastula was developed into the ancestral planula by a 

 proKferation of cells at one pole only, or by an invagination of the 

 cells forming the wall of the blastula at this pole. 



If this be admitted, however, we have no difficulty in deciding 

 that invagination must be more primitive than polar prohferation. 

 Our reason for that decision is that polar prohferation would be 

 meaningless in an adult animal, whereas invagination means, primarily 

 an increase in surface area of a portion of the animal, and secondarily 

 an inbending and the consequent continuous preservation of a 

 cavity between the invaginated cells, which cavity is destined to 

 contain food. 



In fact, as Korschelt and Heider point out, a ciliated animal, 

 progressing forward in one direction, tends to create suction behind 

 it, so that particles struck backwards by the ciha tend to accumulate 

 there ; just as may be observed at the tail of an express train as it 

 dashes past a station. Here then would be sufficient inducement 

 for the tendency to increase and exaggerate the function of ingestion 

 — a function which all the cells of the planula originally must have 

 possessed, — and so we may suppose that an endoderm and an ectoderm 

 would become specialized from an indifferent layer of cells. 



As the endoderm cells increased in number it became necessary 

 that they should find room, and this they did by bending inwards, 

 and so the planular stage is reached. When this stage is repeated 



