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yolk, as in the spiders and scorpions, and certain Crustacea, 
1.e., Eriphia spinifrons, Astacus fluviatilis, Paleemon adspersus, 
and Crangon maculosus, in which there is no metamorphosis. 
The embryo is a Nauplius ; it sheds a Nauplius skin about 
the middle of embryonic life. 
This Nauplius skin corresponds in some respects to the 
“Jarval skin” of German embryologists. 
The recently hatched young of Limulus can scarcely be 
considered a» Nauplius, like the larve of the Phyllopoda, 
Apus and Branchipus, but is to be compared with those of 
the trilobites, as described and figured by Barrande, which 
are in Trinucleus and Agnostus born with only the head and 
pygidium, the thoracic segments being added during after 
life. The circular larva of Sao hirsuta, which has no thorax, 
or at least a very rudimentary thoracic region, and no pygi- 
dium, approaches nearer to the Nauplius form of the Phyllo- 
pods, though we would contend that it is not a Nauphus. 
The larva passes through a slightly marked metamorphosis. 
It differs from the adult simply in possessing a less number 
of abdominal feet (gills), and in having only a very rudi- 
mentary spine. Previous to hatching it strikingly resembles 
Trinucleus and other trilobites, suggesting that the two 
groups should, on embryonic and structural grounds, be 
included in the same order, especially now that Mr. E. 
Billings! has demonstrated that Asaphus possessed eight 
pairs of five-jointed legs of uniform size. The trilobate 
character of the body, as shown in the prominent cardiac 
and lateral regions of the body, and the well-marked abdo- 
minal segments of the embryo, the broad sternal groove, and 
the position and character of the eyes and ocelli, confirm this 
view. The organization and the habits of Limulus throw 
much light on the probable anatomy and habits of the trilo- 
bites. ‘The correspondence in the cardiac region of the two 
groups shows that their heart and circulation was similar. 
The position of the eyes shows that the trilobites probably 
had long and slender optic nerves, and indicates a general 
similarity in the nervous system. The genital organs of the 
trilobites were probably very similar to those of Limulus, as 
they could not have united sexually, and the eggs were pro- 
bably laid in the sand or mud, and impregnated by the sperm 
cells of the male, floating free in the water. 
* « Proceedings of the Geological Society of London,” reported in 
‘Nature,’ June 2nd, 1870. In this communication Mr. E. Billings an- 
nounces the important discovery of a specimen of Asaphus platycephalus, 
showing that the animal possessed eight pairs of five-jointed feet, widely 
separated at their insertions by a broad sternal groove. 
