152 



THE INVERTEBRATA 



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 in- 

 dividuals 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) 



Fig, 112. 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. 



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

 of endoderm is a planula (Fig. 112). It is capable of wide distribution 

 by currents and may live for a considerable period before settling 

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

 enteron, 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 /)o/_y/) and the free w<?^w5«; 

 locomotion usually by muscular action; possessing nematocysts. 



They are divided into the following classes : 



Hydrozoa. Cnidaria, nearly always colonial; typically with free 

 or sessile medusoid phase, arising as buds from the polyp-colony: no 

 vertical partitions in the enteron; medusae with a velum and nerve 

 ring; tentacles of polyp usually solid; ectodermal gonads; and an 

 external skeleton. 



ScYPHOMEDUSAE. Cnidaria in which the polyp stage is incon- 

 spicuous and may be absent altogether: the polyp, where present, 



