352 



PALAEOSTRACA. 



remarkable that, in spite of this position, the innervation of these eyes takes 

 place, not from the corresponding ventral ganglia, but from the brain. Taking 

 into account the close relationship between Limulus and the Scorpiones, 

 which c;ni hardly be denied, we should feel inclined to homologise the lateral 

 eyes of Limulus with those of the Scorpiones. The latter belong,, without 

 doubt, to the pre-oral cephalic region of the body. This homology, however, 

 was disputed by Patten, who regarded the small sensory organ (Fig. 162, s) 

 just described, and discovered by him, as corresponding to the lateral eyes of 

 the Scorpiones. If we consider the distribution of the nerves in the adult 

 Limulus, however, we shall hesitate a little before regarding the lateral eyes as 

 belonging to that thoracic segment in which they appear to lie. Not only do 

 the optic nerves of the lateral eyes come from the brain, but a large branch of 

 the integumental (frontal) nerves arising from the brain runs far back in the 

 lateral parts of the cephalo-thorax. We are thus justified in asking whether the 



Fig. 1G4.— Transverse section through the rudiment of the lateral eye of Limulus (after 

 Watase). om, rudiment of an ommatidium ; c, rudiment of a corneal lens : df, dorsal fold, 

 and vf, ventral fold of the optic rudiment. 



lateral parts of the cephalo-thorax which carry the lateral eyes do not correspond 

 to lateral portions of the pre-oral cephalic region which, during growth, have 

 been secondarily shifted backward. * According to this view, only the middle 

 part of the cephalo-thorax or the glabella, together with the parts of the cheeks 

 bordering on the limbs, could be attributed to the thoracic segments. The 

 position of the facial suture in the Trilobites could also be brought into 

 agreement with this view (p. 337). 



B. The Alimentary Canal. 



The anterior and posterior portions of the alimentary canal arise 



from ectodermal invaginations (stomodaeum and proctodaeinn), 



which only become connected with the enteron after the first larval 



moult. We have already (p. 344) referred to the backward shifting of 



* [Kishinouyk regards the lateral eyes as belonging to the second brain 

 UK-lit, and as true cephalic structures, thus entirely supporting the above.- I 



