(52 COELENTERATA [CH. 



there are radial canals, but as it grows the primary tentacles branch 

 and become bunches of tentacles. The upper surface of the bell is 

 styled the ex umbrella or aboral surface (Lat. ab, away from ; os, 

 oris, the mouth), the lower the subumbrella or oral surface. 



The great mass of the bell is composed of the jelly intervening 

 between the outer ectoderm on the convex side and the endoderm. 

 In this jelly solid strings sometimes appear which give it a firmer 

 consistence. The union of certain of the tentacles by means of a web 

 so as to simulate an umbrella causes the oral cone to resemble the 

 handle, hence the name manubrium (Lat. a handle), by which it is 

 usually designated in a bud of this kind (1, Fig. 25). Just above 

 the circular canal in most Medusae a fold of the outer skin grows in 

 towards the oral cone, so as to form a broad circular shelf: this 

 structure is called the velum (Lat. an awning) (B, Fig. 26; 1, 

 Fig. 27, II). The bud now breaks loose and swims by contractions 

 of the bell, aided by vibrations of the velum. Anyone would now 

 recognise it as a minute jelly-fish, though it really is quite different 

 in many points from the larger and better known animals denoted 

 by that term. Zoologists speak of it as a Medusa, and speak of 

 the stock from which it was budded as a colony consisting of 

 medusoid and hydroid persons, the latter term denoting the 

 ordinary buds which resemble Hydra. The term polyp is an 

 unfortunate one.. It really refers to the swollen end piece of a 

 hydroid person carrying the mouth and tentacles. The early 

 naturalists supposed this to be something distinct from the lower 

 stalk-like portion of the body which they called the "coenosarc." A 

 medusoid is in many respects more highly developed than the 

 hydroid person. The ectoderm cells composing the velum and 

 those forming the lining of the under side of the bell or sub- 

 umbrella are strongly drawn out into processes which are muscular. 

 In the velum these are arranged so as to form two bands running 

 round the edge of the bell or umbrella, one band being in connection 

 with the upper and another with the lower layer of cells composing 

 the fold of ectoderm of which the velum consists. Just, however, 

 where the velum is attached to the bell, its cells upper and lower 

 undergo another and more interesting modification (4 and 5, Fig. 27, 

 II). At their bases a tangle of delicate threads of almost inconceivable 

 fineness appear ; these threads are outgrowths of the cells, but far 

 more delicate than those which already in Hydra we recognised as 

 the forerunners of muscles ; the threads we are now considering are, 

 in fact, nervous in nature, and the tangles of them connected with 



