ON THE DEVELOPMENT OF LIMULUS LONGISPINA. 65 



of the eighth appendage-bearing segment had appeared. Since 

 the preceding' dnv an iiicoTn]>]ete s(']'»finn liad been produced in the first 

 common cœlomic ca\ity, se])arating the ceplialic portion from that 

 of the first appendage-bearing segment (fig. 80). 



On Angnst 16th (19 days after fertilization), three pairs of in- 

 vaginations were visible in the cephalic region. One pair ^vas in the 

 external part of the cephalic lobe (figs. 12, 34, 59, ex. gr.). I shall 

 call it the external cephalic gro(3ve. It is the gro(Dve for the ganglion 

 of the lateral eye. Another ]3air was a small roundish depression, ex- 

 ternal and posterior to the first pair (figs. 12, 35, 59, lat. e.), Jt is 

 the invagination for the lateral eye. The third pair was also a small 

 roundish depression in front of the cephalic lobe near the median line 

 (figs. 12, 34, med. e.). It is the invagination for the median eye. The 

 mesoblast whicli had been divided into lateral halves had now re- 

 united in the cephalic region (fig. 59), and in the second (fig. 33), 

 third, and fourth (fig. 39) appendage-bearing segments. The ninth 

 pair of the cœlomic caA'ities had a})peared (fig. 32). 



On August 17th (20 days after fertilization), one more pair of 

 the grooves had ap])eared in the internal, anterior portion of the cephalic 

 lobe (figs. 36, 38, int. gr.). Tlic new gr<)<)^•e is long arid crescent- 

 shaped. I sliall call it the inter] lal cephalic groove. It is the groove 

 for the brain. The tenth pair of tlie cœlomic cavities liad appeared. 

 The rudiment of the flabellum had appeared as a small, round protuber- 

 ance formed ofhigli, columnar cells (fig. 47). A similar, and undoubt- 

 edly homodynamous organ may be seen in the segment anterior to 

 the segment of the flabellum, though much smaller (figs. 13 and 48). 



On August 18th (21 days after fertilization), the ventral iierve- 

 cord had divided into ganglia (fig. 13). The paired invaginations for 

 the median eye had united (figs. 13, 36, 37). A pair of ronnd pro- 

 tuberances, homodynamous with the flabellum, was found in the seg- 



