274 M.E. Hesse on the Means by which certain Species of 
disappears in such a manner as no longer .to present anything 
abnormal. I cannot say whether this structure is exceptional, 
from the difficulty of preserving for some time alive such small 
Crustaceans, which feed only on the blood of fishes, and, con- 
sequently, of following exactly all the transformations which 
they undergo. It is certain, however, that, at the moment of 
their exclusion from the egg, they do not present this singular 
appendage, and that it is only at the second or third change of 
skin that it makes its appearance in those which acquire it. It 
will be necessary to examine more minutely than I have yet 
done the bodies and branchiz of fishes in order to ascertain that 
the parasites do not attach themselves by this means. I remem- 
ber, however, having seen some young Caligi attached to the 
branchial laminz of a fish by a cord of this kind; so that this 
may be more common than I suppose. 
The form of the young Crustacea attached to their mother is 
perfectly adapted to their situation: it is oval, flat, attenuated — 
at the two extremities, and shaped like a little boat, for the 
purpose of facilitating natation and offering the least possible 
resistance to traction. 
In this state, the cephalic buckler, which is triangular, pre- 
sents at each side of the head two antenne, of greater or less 
length, composed of two joints, and terminated by some rigid 
hairs. The eyes are very large, pressed close together, and 
placed above at the middle of the thorax. 
The abdomen is generally divided into five segments, of which 
the first is the largest ; the hindmost is terminated by two pro- 
cesses furnished with very long and stiff hairs, four in number, 
the two median ones being the longest. 
Beneath, near the antenne, and on each side of the head, are 
the two first thoracic feet, which are more or less developed and 
composed of two articulations, terminated by a very strong 
hooked claw. 
The head is oval, rounded at the apex, and pointed at the lower 
extremity, which is conical, and forms the sucker ; below this, 
according to the more or less advanced stage of development, 
we may perceive a furrow, which is also seen in the adult Trebie 
nd Caligi. 
Below the first thoracic feet those of the second pair are 
observed, armed with two crooked claws: these are followed by 
the third pair, terminated by a single claw. 
The abdominal false feet vary according to the species; they 
generally consist of flat flabelliform joints, fringed with strong 
hairs, and serving for propulsion. 
The ova of Trebie, Caligi, and Pandore are piled together in 
the oviferous tubes like pieces of money in a rouleau. Those of 
