NOTES ON THE EMBRYOLOGY OF LIMULUS. 
555 
both Hexapods and Arachnids two or more urinary tubules 
are developed from the proctodseum (their development in 
Arachnida has not been described) ; in Limulus nothing of the 
sort is found, nor is anything of the kind certainly known in 
the Crustacea. As I shall have again occasion to refer to 
these organs I will leave the enumeration of the horns of the 
dilemma until then. 
The close relationship existing between Limulus and the 
Trilobites has often been insisted upon by naturalists since 
Lockwood first saw a young horseshoe escape from its egg. 
The recent work of Walcott (’81), though I cannot accept his 
interpretations in many respects, serves to show that the 
resemblance between the two are not so great, or rather the 
differences are too great, to warrant a close association of these 
forms. This is more apparent from the discovery of the Ohio 
specimen described and figured by the same gentleman (’84). 
The veteran carcinologist, Henri Milne Edwards (’81), also 
fails to recognise these affinities, though he places his objection 
on different grounds from those that I hold. I do not care to 
enter into a discussion of the problem, but would state that 
according to these results and specimens the Trilobites had a 
series of non-chelate ambulatory limbs extending to the ex- 
tremity of the body. Each limb consisted of a basal joint, 
from which arose an endopodite with six cylindrical joints, a 
three- jointed setose epipodite (exopodite ?), and outside of 
these, just as in the lobster, the branchial organs. These latter 
were filamentary and straight or spirally coiled. With such a 
different appendicular structure it seems to me that we must 
have more than a strained resemblance of dorsal surfaces before 
admitting any close resemblance between the two, though it 
must be said that there is a certain resemblance between the 
four anterior legs of the Trilobite and their relationship to the 
mouth (as restored by Walcott, ’81, pi. vi, fig. 1) and the four 
posterior cephalothoracic limbs of Limulus. 
If we are to accept these resemblances as indicative of a 
homology between the two we must conclude that the first two 
pairs of appendages of the Trilobites have been lost, to say 
