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OBSERVATIONS ON THE EMBRYOLOGY OF LIMULUS. 677 
Anderson School of Natural History, on the anatomy of the adult 
Limulus, I have been able to fully confirm the important discovery 
of Prof. Owen (Lectures,) 1852 and more recently of M. Alphonse 
Milne-Edwards* relative to the sheathing of the nervous cord and 
its branches by a system of arteries, and I would here bear testi- 
mony to the accuracy of Edwards’ drawings and descriptions. 
Moreover I have been able by a study of living Limuli, beautifully 
injected by Mr. Bicknell by the kind permission of Prof. Agassiz, 
the director of the Anderson School, to extend still farther the 
anatomical researches of Milne-Edwards. With Mr. Bicknell’s 
aid I have ascertained the existence of still smaller arterial twigs, 
on the peripheral subcutaneous portion of the body, than indicated 
by Milne-Edwards, and have made out the existence of an exten- 
Sive series of closed vessels in the- respiratory abdominal feet. 
For this I was prepared by a study of the respiratory lamelle, 
which, in the arrangement of their chitinous septa, may be closely 
homologized with the gills of Amphipod Crustacea, as observed in 
living specimens without injection. 
With the new information afforded us by A. Milne-Edwards, re- 
garding the relations of the nervous cord with the ventral system 
of arteries, and the remarkably perfect circulatory system, so much 
more highly developed than that of any other Arthropod, I should 
no longer feel warranted in associating Limnlus and the Merosto- 
mata generally with the Branchiopoda, but regard, them, with the 
Trilobites, as forming perhaps a distinct subclass of Crustacea. 
Certainly if we consider the relations of the anatomical systems 
to the walls of the body, the disposition of the segments forming 
those body walls, and the nature of the appendages, Limulus 
is built on the crustacean type. Because its nervous cord resem- 
bles that of the scorpion, and its circulatory system is more 
perfect than that of any Arthropod we know, this is no reason for 
assuming that it is not a Crustacean. On the same ground Cera- 
todus is not a fish because it has the lungs of a reptile, nor is 
Ornithorhynchus a Saurian because it has the shoulder girdle of a 
Saurian.+ I have, moreover, shown that some important features 
in the embryology of Limulus are like those of the scorpion and 
‘the hexapodous insects, the ‘‘amnion” of Limulus apparently 
being homologous with that of the insects. 
* Recherches sur Anatomie des Limules. Annales des Sc. Nat., 
ti have been reminded by Professor Wyman of this peculiarity in eatin: 
Meckel. 
