THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. XIV. MARCH, 1893. No. 3. 



Hiol<i(^ical Descriptions of (Vrtaiii ('oiiiinoii Hydroid 

 Animals. 



By H. L. OSBORN, 



ST. PAUL, MINN. 

 [with KRONTISPIICCE.] 



In the chapter on the Protozoa (A. M. M. Journal. 1S92) it 

 was shown that a great variety of animal form is possible by vary- 

 ing the t'orm of a single cell aiul its (liferent parts, but none of the 

 Protozoa attain any considerable size. Now, in the competition 

 of animal life size is an advantage to its possessor, other things 

 being equal. Anvverv considerable increase of size is impossible 

 in a single cell, because, as pointed out by Herbert Spencer, the 

 solid capacity outruns the surface area, and thus, in increasing 

 sizes, the points where food can be taken in soon become too few 

 for the bulk. Hence, to secure the advantages of size, cells are 

 forced to club together and form multicellular bodies. We find 

 in the coclenterate animals the simplest type of multicellular bodies, 

 many of which are easilv accessible to large numbers of students, 

 antl this present article is written as a key to the cellular structure 

 of some of the representative members of the group. As many 

 of the readers of this, /f>/^r//a/ live near tiie ocean or visit it dining 

 the siunmer. I trust thc\' will find it possible to verify upon speci- 

 mens they can find some or all of the points I shall speak of. 



Hints for Practical Study of Hvdkozoa. 



A great deal can be done in the study of these animals at very 

 little cost for appliances, but, of course, the best work requires 

 skill and patience and a good compound microscope. 



First, in collecting specnnens of marine Hydrozoa from a boat 

 or when in bathing, examine submerged sea-weed or scan care- 

 fullv the submerged portions of olijects such as piles of wharves, 

 floats, buoys, drift-wood, and remove with knife parts which ap- 

 pear to promise well. |jla(;ing tiie material thus obtained in a tum- 

 l)Ier with plenty of sea-water, (iive it time to recover from the 

 stimulation voii have caused, and if there be any living hydroids 

 in your capture they will presently reward you by expanding; or 



