39 2 



LOWER FORMS OF MARINE LIFE 



example of the alternation of generations. The so-called corallines, or Hydro- 

 corallina, which resemble the true corals in possessing a calcareous skeleton, display 

 a very similar, although in some respects a more complicated type of growth and 

 reproduction, many of them producing free-swimming medusas. The true medusas 

 or jelly-fishes, on the other hand, constitute a group, the Medusae, in which the 

 eggs formed by one medusa develop in many cases directly into another, without 



the intervention of a stationary 

 stage, such a fixed stage having 

 in all probability, as in the case 

 of the tadpole stage of certain 

 frogs, been suppressed. In the 

 typical group of these medusas, 

 which constitute the ordinary 

 jelly-fishes, or sea-nettles, such as 

 Aurelia and Cotylorliiza, there is, 

 however, a fixed stage. In the 

 former of these the fertilised egg 

 of the medusa develops into a 

 hollow embryo, covered with fine 

 hairs (cilia), which eventually 

 settles down in the form of a 

 small polyp provided with diges- 

 tive organs, and carrying sixteen 

 tentacles. During this so-called 

 ' scyphistoma ' stage the polyp 

 undergoes transverse fission, and 

 is divided into a number of 

 segments, each of which re- 

 sembles a saucer with a scalloped 

 margin. These, which then con- 

 stitute the ' ephyra ' stage, 

 separate and swim away ; taking 

 the form of a disc with eight 

 bifid arms. To make a long 

 story short, the spaces between 

 these arms are filled up, certain 

 other changes take place, and the 

 ' ephyra ' eventually becomes a 

 fully developed medusa of the Aurelia or Cotylorliiza type. 



With the exception of a number of fresh-water types, the great 

 group of sponges constituting the group Porifera — of equal value with 

 the Coelenterata — are marine organisms of low grade, which are to be found at all 

 depths; the deep-sea types being those with calcareous or wholly flinty skeletons 

 in place of horny ones. Of those with flinty skeletons the most graceful is the 

 well-known Venus' flower-basket (Euplectella aspergillum), inhabiting the seas 

 around the Philippines at a depth of from 60 to 100 fathoms. The beauty of the 



VENDS FLOWER-BASKET. 



Sponges. 



