COELENTERATA 



131 



bell is greatly developed by the substitution of a type of cell in which 

 the muscular processes form a long striated fibre while the epithelial 

 part is greatly reduced; such a cell is capable of rapid rhythmical 

 contraction. The nervous system may be partially concentrated to 

 form a nerve ring and well-defined sense organs occur in connection 

 with this. In this phylum, the lowest of the Metazoa, the gametes 

 are of the type which is found throughout that great animal division ; 

 the maturation divisions make their typical appearance here. Eggs 

 and spermatozoa respectively are nearly always borne by different 

 individuals or colonies. After fertilization the egg segments by equal 

 divisions until firstly, a single layer of cells (ectoderm) arranged to 

 enclose a central cavity constitutes the blastula. Then, by the migration 

 of cells into this cavity, it becomes filled up with tissue (endoderm) 

 while the ectoderm becomes ciliated. Such a larva with a solid core 

 of endoderm is a planula (Fig. 1 10). It is capable of wide distribution 



Fig. no. Development of a hydroid polyp. After Merejkowsky. A, Forma- 

 tion of endoderm in the blastula, by budding from the pole. B, Planula 

 with solid core of endoderm. C, Appearance of enteron; endoderm cells 

 beginning to arrange themselves as a single layer. 



by currents and may live for a considerable period before settling 

 down. A split appears in the endoderm, the first appearance of the 

 gastrovascular cavity, and the larva sinks to the bottom and attaches 

 itself by one end. At the other end a mouth and tentacles appear and 

 the creature becomes a polyp. There are a few exceptions to this in 

 the phylum in which the egg develops directly into a medusa. 



SUBPHYLUM CN ID ARIA 



Coelenterata referable to two types, the fixed polyp and the free 

 medusa ; locomotion usually by muscular action ; possessing nemato- 

 cysts. 



9-2 



