CNIDARIA 89 



stitial cells and often epithelial muscle fibres aud nerve cells. The mes- 

 oglea is skeletal in function and a secretion of the two cellular layers; 

 in the Hydromedusae it remains non-cellular and usually thin, but in the 

 other Cnidaria, cells migrate into it from the ectoderm and it often be- 

 comes very thick, forming the jelly (Fig. 131, 3). The gastrovascular 

 space (Fig. 147) is cylindrical in the hydroid; in the medusa it is a 

 branched cavity which forms a system of canals. Food is taken into the 

 mouth ; in the gastrovascular space it is digested and the products of diges- 

 tion are caused to circulate throughout the body of the colony by the action 

 of the entodermal flagella or cilia. The sexes are separate in Cnidaria, 

 with some exceptions, but usually not dimorphic. The sex cells arise in 

 the ectoderm in the lower and in the entoderm in the higher forms. 

 Locomotion is accomplished by means of the muscle fibres which are the 

 inner projections of ectoderm or entoderm cells. The sessile hydroids 

 and Anthozoa move their tentacles about actively and can retract and 

 extend the body; the medusae swim slowly through the water by means 

 of the muscle fibres in the velum or in the subumbrella. The nervous 

 system consists of a plexus of nerve cells and fibres among the muscle 

 fibres, some of which, in the medusae, form a double ring in the outer 

 rim of the umbrella and in the acraspedote medusae a rudimentary gan- 

 glion at the base of each sense organ. The muscles may also be stimu- 

 lated directly and without the intermediary of nerve cells or fibres. 

 Special sense organs are absent in hydroids and the Anthozoa : in medusae 

 they are present in the margin of the umbrella and may be either visual 

 (ocellate) in function or equilibrial (vesiculate). 



History. Aristotle was acquainted with many cnidarians, especially 

 with actinians and medusae which he named Acalephae and Cnidae, the 

 latter term referring to the stinging power of the animals. During the 

 succeeding ages and down to about the middle of the eighteenth century 

 the animals were observed and figured by a number of naturalists, but 

 little or no exact knowledge of them existed. They were called either 

 plants or plant-animals (zoophytes) and were often considered the con- 

 necting link between the plant and animal kingdoms, a belief that has 

 not entirely disappeared in some localities even down to the present day. 

 Polyp stocks and corals were generally held to be plants, the individual 

 animals being called the flowers. When, however, Trembley in 1744 

 demonstrated the animal nature of Hydra and Peyssonnel in 1753 that of 

 corals, a new era began in the study of cnidarians, and in the following 

 years a large number of them were accurately figured and described by 

 Ellis, Pallas, O. F. Miiller, and others. The relation of the polyp to 

 the medusa was, however, still for a long time to be entirely unknown. 

 Cuvier in 1799 was one of the first to study the anatomy of the medusa 



