68 COELENTERATA 



this nerve net protoneurons . There are sense organs for equilibra- 

 tion, touch, visual sense and gustation. 



Reproduction is asexual, by fission and budding; and sexual, 

 by ova and spermatozoa. Hermaphroditism and separate sexes 

 are both found. Many species of Coelenterates give off medusae 

 when not a month old. Their life cycles may be completed in three 

 months. Sagartta completes its life cycle in about 15 months. The 

 sea anemones may undergo pedal " laceration," which is asexual 

 reproduction. 



Size. — Hydrozoa are mostly of small size. The Scyphozoan 

 jelly fishes are larger, Cyanea arctica reaching a diameter of 12 feet 

 with tentacles nearly 100 feet long. The largest reef anemone, 

 Discoso?na, an Actinozoan found in the Mediterranean Sea, reaches 

 a diameter of 1 feet. 



Habitat. — A little hydroid lives in the mouth of the tube of the 

 worm Sabella. Stylactis^ another hydroid, lives on the skin of the 

 rock perch, Minous minot. Stylactis vertnicola attaches to the worm 

 Aphrodite at 2,900 fathoms. Scyphozoa have been found at a 

 depth of 2,000 fathoms. 



Regeneration. — The remarkable ability of Hydra to regenerate 

 has been known since the experimental work of Trembley in 1744. 

 The hypostome with the tentacles will produce an entire new Indi- 

 vidual, and as many as seven heads have been produced by splitting 

 the animal anteriorly. Hydra may be turned Inside out and become 

 normal in a short time. It was at first thought that the ectoderm 

 cells were transformed into endoderm. This Is not so, however. 

 The animal either turns Itself back or else the inturned ectoderm 

 disappears and new ectoderm forms from the lips downward, cover- 

 ing the endoderm. 



Interesting experimental work has been done by several investi- 

 gators with various Scyphozoa Including Aurella. They regenerate 

 remarkably when segments are cut out. 



Fossil Relatives. — Hydrozoa are found as fossils from the Cam- 

 brian to the present. Scyphozoa are 99 per cent water and so few 

 traces remain In the rocks. Lithographic slates, found In the Juras- 

 sic strata of Bavaria, show the impressions of the thin soft bodies 

 or tentacles of jelly fishes. Sometimes the digestive cavity was 

 filled with sand and covered by other mud or sand before the body 

 of the jelly fish disintegrated and so the outline of the containing 

 cavity was preserved. Actinozoa or Anthozoa are found from the 



