HIGH SCHOOL ZOOLOGY. 



247 



which, in some cases, differ very much from the ordinary polyp 

 in sliape, and may even be detached and swim off as Medusce 

 (Fig. 170). There is then an alternation between the Hydra- 

 form, multiplying by buds, and the Medusa-form multiplying by 

 eggs. Some Meduspe, however, thelarger jelly-fishes (Fig. 180), 

 do not come from Hydroid colonies, but their eggs give rise to 

 tube-like larvae, which undei-go multiplication by division into 

 young Medusae, related to each other like a pile of saucers. In 

 such, higher forms of Hydroids an abundant layer of gelatinous 

 connective tissue (mesoderm) separates the endoderm from the 

 ectoderm. 



6. The class Actinozoa (exclu- 

 sively marine) derives its name from 

 the Sea-anemone — Actinia — (Fig. 

 1?1). These creatures attain a 

 considei'able size, and are often very 

 brilliantly coloured. They differ 

 from the Hydroids by having the 

 mouth-end of the tubular body 

 turned in as a stomach-sac, which is connected by mesenteries or 

 septa to the outer tube (Fig. 182). The chambers between 

 the septa open above into the tentacles, 

 which are separated from the mouth by 



¥\g. 180.— Higher Jelly-fish. 

 {Aarelia aurita). 







an intervening disc. The Sea-anemonies 

 have no skeleton, and do not form 

 colonies, but allied forms give rise by 

 budding, or division, to colonies, v/hich 

 may be arborescent or massive, and in 

 which a skeleton or corallum is a 

 marked feature. The corallum may 

 be confined to the axis of the common 

 flesh (ccenosarc), which unites the indi- 

 vidual polyps, as in the Fan-corals and Fig. isi.-A Sea-anemone {AcH- 

 the Red Coral of commerce (Fig. 183), She t^nuc^s • d/n^S oAhi 



or it may invade the polyps tliem- 



disc ; e, the wall ; f, the base. 



