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 diflBculty 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 branchiae of fishes in order to ascertain that 

 the parasites do not attach themselves by this means. I remem- 

 ber, however, having seen some young Cali(/i attached to the 

 branchial laminae 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 oflfering the least possible 

 resistance to traction. 



In this state, the cephalic buckler, which is triangular, pre- 

 sents at each side of the head two antennae, 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 antennae, 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 Trebia 



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 Irebice, Caligi, and Pandorce are piled together in 

 the oviferous tubes like pieces of money in a rouleau. Those of 



