i62 AN INTRODUCTION TO ZOOLOGY. 



Many forms are common on our coasts, under the 

 popular name of corallines. Tubularia (Fig. 36) has 

 long straight tubes containing single polyps. Anten- 

 nularia has straight tubes with short verticillate 

 branches : Sertularia branches in one plane, i.e., is 

 bilateral like a leaf or feather ; Pltimularia, the sickle 

 coralline, has feather-like branchlets arranged on a 

 spiral axis. Oampanularia has bell-shaped heads 

 borne on an irregularly branching stalk. This order 

 presents the fixed generative buds called blastostyles. 

 The medusae, when free, are usually rather small. 

 They have a nerve ring round the disc, and sense- 

 organs connected with it. These sense-organs are 

 not especially protected, and the order has therefore 

 received the name of Gyrano;plithalmata, or naked-eyed 

 medusae, in distinction from the Steganophthalmata 

 {covered-eyed medusae), i.e., Acraspeda or Scypho- 

 medusce, described below. The most easily distin- 

 guished feature about the medusEe, however, is the 

 existence of a fold, turned in along the margin. This 

 is called the velum or craspedon (i.e., veil or 

 border), and from its presence this order has also 

 received the name of the Craspeda. The order 

 contains not only forms with a hydroid stock, but 

 also some forms {Trachymedusce) which are developed 

 directly from the larva, and never have any hydroid 

 stage. 



The Siphonophora have carried the principle of the 

 division of labour to such an extent, that their polyps 

 are hardly to be recognised as such, while the medu- 

 soids rarely become free. The whole colony forms a 



