THE MICROSCOPE. 67 
species of the same genus, Stephanomia, from the Atlantic. When 
irritated, these organs can be made to throw off a colored fluid which 
calls to mind the discharge from the surface of the bracts of Agalma. 
Haeckel says,* “‘ When a quietly floating Forskalia is touched, it sud- 
denly discharges the contents of the chromadenia | pigment glands], 
and makes the surrounding water dark and intransparent.” KOlli- 
ker also mentions this discharge of coloring matter from the “ Fiih- 
ler,” and the same is described by Leuckart. These last mentioned 
naturalists regard the discharge, however, as due to a rupture of the 
wall of this organ rather than an emission from an opening, and the 
former says of it,t ‘“ Ohne Zweifel ist diese Substanz ein Excretion- 
stoff, doch wird ohne genauer Kenntniss ihrer chemischen Beschaff- 
enheit nichts Niheres iiber ihre Bedeutung beizubringen sein.” 
Haeckel offers the following explanation of this phenomenon in 
Forskalia: “The execretion of the pigment-masses and the darken- 
ing of the water by it have probably the same physiological function 
as in the Cephalopoda—to protect the attacked animal from its perse- 
cutors, and facilitate the capture of food animals.” The suggestion 
may apply also to the pigment glands of the bracts of A. clausi, and 
the situation of these bodies on the bracts is very favorable to such 
a function. In no case, however, have I observed that the amount 
of the discharge is large enough to completely darken the water, 
although it is not impossible that the fluid thus emitted may be of a 
poisonous nature and therefore fatal to the small animals, the food 
of the Physophore, with which it comes in contact. The absence of 
these glands on the nectocalyces and its presence on those organs 
adjacent to the tentacles is significant, and would seem to have some 
weight in our judgment of their physiological function. 
Although the hydrophyllia or bracts of the Siphonophores are 
so large and prominent in all medusz, where they are present, no 
satisfactory explanation of their function has yet been proposed. 
They are commonly stated to act as coverings to the polypites and | 
gonophores—a function which they may in part perform. In many 
cases, also, as in the Anthophysidz, they assist in locomotion, as I 
haye elsewhere shown in au Athorybia from Dry Tortugas, Florida. 
It is not impossible, however, that they may be regarded as 
organs of respiration, for which they seem from their form well 
suited. In those genera where they are wanting, we generally have 
an enlarged float, and the possibility of aerial respiration, as in 
Vellidee, Rhizophyside, Physaliad, ete., or as in Physophora and 
* Report on the Challenger Siphonophore. 
+ Siphonophoren von Messina, p. 8. 
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