43 
the animal ean double himself up and use this part of his body as a 
powerful instrument of propulsion. Those of the head and thorax 
were welded together into one piece, called tke ‘‘ carapace,” having, 
however, a slight depression which distinguishes the head from the 
thorax. Of the abdominal somites Mr. Turner took the third 
as a typical segment, which he fully described, and also explained 
that all the other somites were formed upon this plan, but were 
variously modified. Referring to the internal organisation of the 
lobster, Mr. Turner said the shell was, of course, filled partly with 
muscular tissue, by means of which the animal movel about, and 
which was the indigestible stuff people ate when they partook of a 
lobster supper. He described the pericardium, the heart, the 
respiratory apparatus, the stomach, the nervous system of the 
articulata and the vertebrata, and the reproductive organs, and with 
this he concluded his sketch of the lobster as an example of the 
general plan upon which the crustacea were constructed. Of the 
other groups of crustacea, one need only glance at some of them to 
see that, although constructed upon this same general plan, they 
departed very widely from the type he had selected—some more, 
others less, to suit their various modes of life, and the conditions by 
which they might be surrounded. 
Mr. Turner, in referring to the structure of the Trilobite, 
said that of its external organization, we know nothing; but 
we might fairly presume that—to some extent, at any rate 
—it resembled that of living crustacea, and was apon the 
same general plan, although, perhaps, ditfering widely in details. 
The only living animal whose form approaches that of the Trilobite is 
the Limulus, or King Crab of the Eastern Seas. This animal has a 
horse-shoe-like carapace, sword-like tail, and limbs, some of which 
are used for swimming, others for jaws. lLimuloid forms are 
found in the carboniferous rocks. The most striking feature in the 
outward structure of the Trilobite is that from which the family 
name is derived, namely, its longitudinal trilobation or division into 
three lobes. This trilobation is common to all Trilobites, but is 
more distinct in some genera than in others. Besides this longi- 
tudinal division by furrows more or less deep, the exo-skeleton is 
divided into three lateral parts—the head, thorax and pygidium, or 
tail. These large divisions are composed of rings or segments 
which vary in number in different genera and species. Those of 
the head are united into a kind of carapace or cephalic shield; the 
segments of the thorax and tail being distinct by means of furrows, 
as in the abdomen of the lobster. The middle lobe of the Trilobite, 
answering to the back of the lobster, is called the axis, and the side 
lobes the pleura. The posterior angles of the cephalic shield are 
frequently prolonged into spines of a greater or less length, as are 
