150 THE INVERTEBRATA 



occur in the rest of the coelenterates. A sense cell is shown in 

 Fig. 108 s.c. It has a rod-like process projecting from the surface and 

 at its other end it ends in slender branches which join with those of 

 the nerve cells. Such sense cells respond to touch and probably also 

 to light and chemical stimuli. If a polyp is touched with a wire the 

 disturbance is transmitted in all directions by the nerve net and 

 results in a general contraction of the muscular system, which may 

 last for long periods. In some cases coelenterate polyps are only 

 capable of expansion in the absence of light. 



This "nerve net " is the most primitive type of nervous system. The 

 cells which compose it differ from the nerve cells of higher Metazoa 

 in their simple structure, and above all in the fact that they are ar- 

 ranged in a diffuse fashion, and not aggregated along particular lines. 

 This is at any rate true for the most primitive polyps : in the medusae 

 and the more differentiated polyps the nerve cells tend to con- 

 centrate in special parts but not in such a fashion as to form any kind 

 of a central nervous system. 



Much of the interest of the coelenterates lies in the conflict between 

 the two modes of life, an easy sedentary existence and a wandering 

 or rather freely-drifting life which demands a larger measure of 

 activity and a greater elaboration of structure and physiological de- 

 velopment. The two types of individual which correspond to these 

 modes of life are the Polyp and the Medusa. There are large divisions 

 of the coelenterates in which only one type is present, while in the 

 others they may even be united in the same species and the same 

 colony of that species. A survey of the phylum is very largely con- 

 cerned with the variations of these types and the combination of them 

 in the life histories of the different coelenterates. 



The polyp (Fig. 1 1 1 A) is an attached cylindrical organism with a 

 thin body wall consisting of two single layers of ectoderm and endo- 

 derm separated by a narrow structureless lamella. At the free end an 

 oral cone occurs and at its apex the mouth opening into the enteron. 

 The oral cone (in the Hydrozoa) is surrounded by a number of 

 tentacles, which are usually very extensible and armed with batteries 

 of nematocysts, by which the living animals, on which the coelenterate 

 feeds, are caught. Tentacles contain a prolongation of the endoderm 

 which may form a tubular diverticulum of the enteron or a solid core. 

 The medusa (Fig. 1 1 1 C) is a free-living organism differing from the 

 polyp in the great widening of the body, especially along the oral 

 surface, and the restriction of the enteron by the increase in thickness 

 of the structureless lamella on the aboral side of the endoderm, so 

 that while a central ^^^^nc cavity remains, the two endodermal surfaces 

 have come together peripherally to form a solid two-layered endoderm 

 lamella except along certain lines, where the canal system is developed 



