MORPHOLOGY OF BEAIN AND SENSE ORGANS OP LIMULUS. 57 



of the stomodseal nerves. I have made this change in the 

 typical insect nervous system shown in figs. 54^ 55, and 56, 

 and the transformation is very suggestive. With only slight 

 modifications of the nerve-rings back of the oesophagus, the 

 pontes stomodaei, with the frontal and the lateral stomodseal 

 ganglia, are seen to form a circumoral nerve-ring ; and by 

 continuing the nerves, uniting the lateral sympathetics, p.p. w,, 

 till they meet on the dorsal side between the body and the head 

 our ideal insect embryo appears like the larva of Lopadorhyn- 

 chus, or of an advanced trochosphere; the stomodseal nerves 

 forming on the sub-umbrella a system of circumoral nerve- 

 rings, such as we might expect to find in a Coelenterate. The 

 labrum is carried forward by the evagination of the oesophagus 

 to the umbrella, and its nerves now appear as umbrella nerves 

 originating from a circumoral nerve-ring. 



This transformation renders the homology of the labrum 

 with the pre-oral antennae of Annelids, as suggested by 

 Korschelt and Heider, very plausible, and at the same time it 

 explains their remarkable innervation from the stomodseal 

 ganglia. The connection of the anterior ends of the sympathetic 

 nerves of the trunk is not known. If they are connected with 

 the system of stomodseal nerves, as indicated in the figures, 

 they might be regarded as sub-umbrella nerves, drawn out to 

 their present extent by the growth of the trunk. The 

 median sympathetic nerve would then be antemeric with the 

 unpaired stomodseal nerve, as in fig. 54. It is obvious, on 

 inspection of the figures, that the invagination of the sub- 

 umbrella must have been greater in front than elsewhere, 

 for the lateral stomodseal ganglia are carried only to the edge 

 of the permanent mouth, while the frontal ganglion is carried 

 far into the oesophagus, bringing the labrum up to the anterior 

 border of the mouth. 



The advantages of this view are obvious. It afi^ords a means 

 of identifying, approximately, the " trochosphere " in the 

 cephalic lobes of Arthropods; explains, among other things, the 

 anomalous innervation of the labrum, and the limitation of the 

 stomodseal nerves to the ectodermal portion of the alimentary 



