104 MANUAL OF ZOOLOGY SECT. 



cavity, and a marginal and four radial canals, and a velum 

 is universally present. But lithocysts are not present in all, 

 their place being taken by specks of red or black pigment 

 the ocelli or rudimentary eyes at the bases of the ten- 

 tacles. The number and arrangement of the tentacles is 

 subject to considerable variation. The gonads are some- 

 times, as in Obelia, developed in the radial canals, some- 

 times in the manubrium. In size the medusae range from 

 about i up to 400 millimetres (16 inches) in diameter. 



In many of the zoophytes, however, the medusae never 

 become detached from the colony, developing the ova and 

 sperms without becoming free. 'In such cases the charac- 

 teristic medusa structure is more or less imperfectly de- 

 veloped, and in many forms is not at all recognisable, the 

 buds corresponding to those which in Obelia give rise to 

 medusae merely developing into rounded outgrowths termed 

 sporosacs, in the interior of which the ova and sperms are 

 formed. 



The reproductive buds are not in all cases formed, as in 

 Obelia, on distinct, peculiarly modified, mouthless zooids. 

 In many instances, whether they are destined to give rise 

 to medusae or sporosacs, the buds spring directly from the 

 ccenosarc, or from the ordinary zooids. 



A small group of Hydrozoa the Hydrocorallina in- 

 cluding the Millepores {Millepord) and Stylaster, form 

 colonies, the supporting material of which, instead of being 

 chitinoid, is of calcareous and stony character, like the 

 substance of a coral. 



The colonies of Hydrozoa are not in all instances at- 

 tached, like those of Obelia and the other hydroid zoo- 

 phytes. In one large order, the Siphonophora, the colonies 

 of zooids float or swim freely in the sea. In some Siphono- 

 phora there are no organs for active locomotion, and the 



