AGASSIZ. NATURAL HISTORY OF THE UNITED STATES. 485 
ness of the wall. There are two kinds of liydrae in one and the same community ; 
the large ones, with very few, and generally only four or five, and seldom six, short 
tentacles, around the head, terminating in a more or less spheroidal knob, supported 
by a short peduncle, are fewer in number ; the smaller ones are much more 
numerous, and more active. The latter differ chiefly from the larger hydrse, in 
having tentacles scattered upon the whole length of the stem, like Halocharis ; but 
instead of being gradually larger from base to summit, the reverse is the case with 
the tentacles of the small hydras of Millepora, the lower ones being the larger, and 
those near the summit growing gradually smaller and smaller. The knobs of all 
these tentacles are chiefly made up of larger lasso-cells, the largest of which have 
a very long thread, remarkable for the enlargement of its spiral band, at a great 
distance from the bulb.” 
Organs of reproduction were not discovered. If these be exter¬ 
nal, and if Millepora want a proper gastric sac, then this genus must 
undoubtedly be placed apart from the Actinozoa, though whether 
with the Hydroids or not may be looked upon as an open question. 
It will also be disputed whether the analogy should be extended to 
all the other Tabulata. Some there are who may feel disposed to 
ask, whether the soft structures seen by Professor Agassiz on the 
skeleton of Millepora were not, in reality, parasitic forms, and, before 
assenting to the wholesale alteration in our systems which he de¬ 
mands, crave for some repetition of his observations. Scepticism, 
in science, is often not only allowable, but desirable; and a man of 
science so eminent as Professor Agassiz would be one of the first to 
acknowledge the liability of all, even of the best, observers to error. 
In further corroboration of his views, Professor Agassiz goes 
on to describe the skeleton of Millepora alcicornis , Pocillopora dami- 
cornis, and Seriatopora subulata, from the “ peculiar characteristics” 
of which latter genus he infers, “that the Corallaria rugosa of 
Milne-Edwards are more likely to have been Hydroids than true 
Polyps.” 
“ At the very earliest stages of growth recognizable on the corallum, the young 
cell possesses a columellar projection, such as is so prominent in the older cells. 
Originally, then, these young cells have the form of inverted, truncated cones, 
which finally deepen and become parallel sided, but as they do this the central 
columella rises, and at the same time, usually, four perpendicular partitions, at 
ninety degrees from each other, are thrown out from the axis to the periphery, in 
such a way as to produce four cavities around the axis. After the cell has attained 
a depth usually equal to its breadth, a transverse diaphagmic partition is developed, 
and then another chamber, or rather a fourfold cavity is formed, to be eventually 
partitioned off like the preceding one, and so on until the end of the existence of the 
hydra.” 
These observations are, at least, useful in showing the necessity 
of examining fresh examples of corals in different stages of growth. 
Eor their hard parts are much more delicate than, at first sight, 
would appear, and require, therefore, careful manipulation. “ It is 
seldom that in dried specimens of the corallum [of Millepora alci- 
cornis ] the natural smooth surface can be studied with confidence, 
on account of the extreme delicacy of the spongiform mass of most 
recent growth. It is impossible, even with the utmost care, to handle 
