282 
MOTES AND MEMORANDA. 
a rectangular body, and contained in their interior a yellowish brown 
matter, with globules of a deeper colour, and resembling the ordinary 
endochrome of the diatoms. At each of the four angles was a long 
hyaline arm, of great mobility. Seen in profile, the body presented 
the form of an elongated oval. M. Guimard, who considers them to 
be parasites, adds that they were endowed with extraordinary agility, 
and by means of their long and flexible appendages explored all the 
parts of the frustules. A woodcut accompanies the paper.* 
Mode of Development of the Tentacles in Hydra. — Mr. Meresch- 
kowsky recently expressed the opinion that the fundamental number of 
the organs in the Hy droids (that is the number which enters into the 
composition of all the other numbers) was not four, but two, arriving 
at this opinion partly from facts which had shown him that the appear- 
ance and sometimes the disappearance of the organs takes place so 
that they appear or disappear simultaneously two at a time. Observa- 
tions on the mode of production of the tentacles in Hydra vulgaris 
and H. oligactis, he considers, serve to confirm this ojunion, and to 
establish a general law which governs the formation and the order of 
appearance of every organ in this class. 
The order of the appearance of the tentacles he finds as. the result 
of his observations (which extended to three complete pairs) to be 
this : — 
The first two appear at the same time and (what is especially 
remarkable) are arranged opposite to each other ; the others also appear 
in pairs, and are also arranged opposite one another ; they do not, 
however, appear together, the second tentacle of each pair always 
appears later than the first, and this retardation is much greater in 
the third pair than in the second. 
This curious mode of appearance of the tentacles in the genus is, 
so far as known to the author, peculiar to it, and does not occur else- 
where among the Hydroida, in which we observe three types of de- 
velopment, viz.: — (1) Appearance in pairs ; (2) Appearance by four at 
a time ; and (3) Appearance of all the tentacles at once, as for 
example in Tuhularia. This exceptional case would serve very well to 
explain the fact (which is also exceptional) that in Hydra we very often 
observe the number seven, which does not accord with the formula 
2 X ^2- that in general characterizes all the Coelenterata. In fact, if 
the sixth tentacle does not appear until long after the fifth, we may 
expect that in the fourth pair of tentacles the seventh will appear 
earlier than the eighth, and that this last will be delayed much more 
than was the sixth. It is in this way that we find a variable number of 
tentacles in the different species of Hydra, sometimes six, seven, eight, 
or even more. It may well be supposed that the individual sometimes 
dies before having had time to acquire an eighth tentacle. But there is 
no reason for thinking that the number of tentacles in Hydra is subject 
to such variations that it cannot be governed by any law. We may 
easily see that the facts are subjected to a general law, although, 
owing to their great complexity, the law does not strike one at once, 
* ‘ Bulletin cle la Societe Beige de Microscopic,’ vol. iv. p. 304. 
