60 WILLIAM PATTEN. 



a short distance only into its interior. The distal end of the 

 tube is not specialised, and shows no trace of the eye that is 

 subsequently developed from it. These two tubes soon unite to 

 form a common tube opening by a large pore, situated some 

 distance in front of the brain (PL 3, fig. 24) . Two bands of ecto- 

 derm, in which there is a shallow groove, lead on either side to 

 the point where the original invaginations of the eye-tubes were 

 situated (fig. 24, c. m. e. t.). The distal end of the tube is now 

 slightly swollen, and constitutes the "anlage" of the median 

 eye. The paired invaginations of the median eye- 

 tubes in Limulus, therefore, are homologous with 

 the invaginations on the second segment of Scor- 

 pions, with which they agree in their relation to the other in- 

 vaginations. They are, however, much smaller than those of 

 Scorpions, and instead of advancing backward and inward over 

 the cephalic lobes, they advance forward and inward to a 

 point in front of the brain, where they unite to form a single 

 invagination extending right into the yolk, and apparently 

 having no connection with the cephalic lobes. A comparison 

 of figs. 24 and 25 and the sections shown in figs. 41 and 42 

 will show that the opening into the median eye-tube really has 

 much the same position in Limulus as in the Scorpion, so that 

 there can be no doubt that the median eyes in these two forms 

 are homologous. The strange position of the median eye-tube 

 in Limulus is caused partly by the forward migration of the 

 invaginations, and partly by the shortening of the cephalic 

 lobes, which brings all the invaginations into a nearly straight 

 transverse line, instead of being distributed along a broad 

 semicircular curve, as in Scorpions. Moreover the rapid in- 

 vagination of the semicircular lobes and their backward growth 

 under the rest of the brain have something to do with the 

 apparent separation of the median eye- tube from the brain 

 (figs. 41 and 42). 



A narrow commissure, similar to that seen in Myriapods 

 and in the early stages of Acilius, unites the right and 

 left halves of the cephalic lobes (compare figs. 57, 59, 61, 

 62, c'.). 



