17 ZOOLOGY. 



217. General Characters. 



1. A single system of internal chambers (gastro-vascular 

 cavity} in which digestion and circulation both occur. No 

 ccelom. 



2. Parts radially arranged about an oral-aboral axis. Ten- 

 tacles usually occur at the oral pole (Figs. 80, 83). 



3. A supporting layer or mass (mesenchyma) between 

 ectoderm and entoderm, sometimes without cells. More often 

 cells of various kinds occur, which have migrated from the 

 other layers. 



4. Nettle cells are found in practically the whole group 

 (Fig. 81). 



5. Nerve cells (sensory) and muscle cells both occur. 



6. Reproduction by non-sexual methods is prevalent. This 

 often alternates regularly with the sexual. The individuals of 

 the two generations may be very different in appearance and 

 habits. 



7. Wholly aquatic; chiefly marine. 



218. General Survey. The group of Qelenterata em- 

 braces animals very diverse in general appearance, which may 

 nevertheless be reduced to two types. The first and most 

 primitive is the tubular hydroid type. This is sessile and is 

 essentially a gastrula, at the unattached end of which occurs 

 the mouth, usually surrounded by tentacles. The cavity of 

 the tentacles is continuous with the gastro-vascular cavity 

 (Fig. 79). Of this type we may distinguish two conditions: 

 (i) in which the individuals (polyps) occur singly (Hydra}, 

 or if in colonies, the various individuals have the same form 

 (as the corals} ; (2) colonial forms in which the individuals 

 making up the colony are very different (as the Siphonophora} , 

 embracing open-mouthed nutritive individuals, mouthless re- 

 productive polyps, protective polyps abundantly supplied with 

 nettle-cells, bladder-like supporting polyps, etc. (Figs. 84, 85). 

 The extreme conditions of (i) and (2) are connected by 

 types possessing intermediate degrees of polymorphism. 

 Though the individual polyps are attached, the whole colony 



