LIFE-FORMS OF THE PAST AND PRESENT. 
401 
a large number of genera), it is hardly probable that mere 
branchial feet would serve for their locomotion. 
If, however, each free and movable thoracic segment was 
furnished with a pair of appendages, as among the modern Iso- 
pods, and as is also indicated in the larva of the earliest stages 
of development within the egg of the modern Limulus 
(See Plate XCI., figs. 17-19), then another point is gained 
in our investigation, and we see that the earliest embry- 
onal stages are those which naturally foreshadow the earliest 
and simplest adult forms. In other words, all the immense 
variety of forms in a group are but the expression of the sum 
of the stages passed through by the highest individual in 
arriving at perfection 
Another relation which the Merostomata and Trilobita ex- 
hibit, and upon which much stress has been laid by Dr. Dohrn 
and Prof. Hseckel, is that between these palseozoic types and 
the Arachnida ; particularly between the Eurypterida and the 
Scorpionidse. 
And it is a most significant fact that the earliest Arachnides 
occur as far back as the Coal-measures, where the last of the 
Eurypterida and the Trilobita are also met with. Anyone who 
has examined a scorpion, or is acquainted with its form and 
structure from books and drawings, cannot fail to be struck by 
the remarkable resemblance between it and the Eurypterida, 
even to the arrangement of the appendages, the position of the 
eyes, &c., &c. Indeed, we may very fairly infer that from this 
division of the Crustacea the Scorpionidse of to-day were derived. 
Nor is there any insuperable difficulty in accepting this view 
on sound physiological grounds. The possibility of an animal 
passing through larval conditions, casting aside, at even a 
single moult, its branchiae, and assuming aerial respiration, 
quitting the water and inhabiting the land, changing its ele- 
ment, its diet, its mode of progression, and its entire life, is no 
chimerical speculation. Such cases are familiar to the ento- 
mologist,* the carcinologist,f and even to the herpetologist.^ 
But the acceptance of this proposition does not, as has been 
assumed by these writers, necessitate the removal of the 
Eurypterida from the Crustacea; on the contrary, as Fritz 
Muller well observes, “ If all the classes of the Arthropoda 
(Crustacea, Insecta, Myrapoda, and Arachnida) are indeed all 
branches of a common stem (and of this there can scarcely be 
a doubt), it is evident that the water-inhabiting and water- 
breathing Crustacea must be regarded as the original stem 
* The larval and adult Libellula, Ephemera, &c. 
f Geccircinus ruricola, and other land-crabs. 
X The Batrachia. 
VOL. XI. — NO. XLV. 
D D 
