IS+ AN INTRODUCTION TO ZOOLOGY. 



with ectoderm, whicli hangs down freely into the 

 general cavity of the body. Vertical partitions, called 

 mesenteries, project from the body wall into the 

 enteron ; some of these pass up into the space round 

 the oesophagus, and thus divide that space into a 

 number of chambers, which communicate above with 

 the hollow tentacles. These mesenteries occur in 

 pairs. In the young animal their development shows 

 traces of a bilateral arrangement, and may indicate 

 that the radial structure of the actinia is secondary, 

 and derived from a primitive bilateral type. 



Sense cells are found in the outside layer of the 

 body of the Actinozoa, and superficial nerve cells con- 

 nected with them ; while muscle fibres sometimes 

 appear in the middle layer. The animals are usually 

 unisexual, and the sexual cells are situated on the 

 mesenteries. 



of the sporophore bears on its stamens and pistil respectively 

 the pollen-grains and the ovules, which are the homologues of 

 the cryptogamic spores ; these produce sexual cells, so that it 

 appears to be a sexual plant. It will be seen from the above 

 facts that alternation of generations, though exceptional 

 among animal forms, is the general type of development 

 among plants. The main difference in the development of 

 alternate generations as exhibited by plants is, that it is 

 complicated by the additional feature of the existence of the 

 asexual spore, a link between the two forms with which there 

 is nothing quite comparable in the animal series. 



For details regarding the above facts, see " Text-book of 

 Botany," Prantl and Vines ; Parker, " Lessons in Elementary 

 Biology," Lessons xxix. and xxx. ; also Enoycl. Brit., art. 

 " Beproduction — Vegetable," by S. H. Vines. 



