178 HYDROZOA AND MEDUSAE. 



times be provided for them. The Zooea will leave the parent 

 soon after emergence from the egg, and if carefully looked for 

 may be seen swimming about, but they are mostly tiny little 

 fellows. It will now be possible to record their development, by 

 taking a few every two or three days and examining them under 

 the microscope, making drawings of them at the time, or 

 preserving them as slides for future work. 



I will conclude by referring my readers to the Journal of 

 the Royal Microscopical Society^ Vol. III., Part 6, for December, 

 1883, p. 785, where they will find a paper by me on an improved 

 method for the preparation and mounting of these and other 

 delicate marine organisms. 



1bv^bro3oa anb flDcbue^^ 



By J. B. Jeaffreson, M.R.C.S., etc. 



Plate 18. 



SCARCELY any result of Microscopic research has been more 

 interesting and unexpected than the discovery of the close 

 connection between the Hydroid Zoophytes and the Medusoid 

 Acalephse, or Jelly-fishes. So utterly different are they in size, or- 

 ganisation, and mode of life that they were long considered as 

 separate and distinct creatures ; but it is now discovered that many 

 of the Medusae are really only the sexual apparatus of certain 

 members of the Hydrozoa. 



The Compound Hydrozoa consists essentially of an aggregation 

 or colony of partially independent polypites or zooids, almost iden- 

 tical in structure with the familiar pond Hydra ; but, instead of 

 leading a separate existence like that animal, remaining permanently 

 connected with one another by a common flesh or coenosarc. 

 Each group or colony commences its existence as a free-swimming, 

 ciliated, oblong body, called "a planula," very closely resembling 

 an infusorian, which soon attaches itself by one extremity to some 

 solid object ; and at the opposite end developes a mouth, sur- 

 rounded by a row of tentacles. The mouth opens into a chamber, 



