PLATE II. 



Development of Xiphosur.v {Limuhis Poli/pliemus). Figures from A. S. Packard, Anton Dohrn, 



and Alexander Agassiz. 



ocl. Ocellus. 



a. Inner egg-menibi'aue. 



V'll. Seventh pair of appendages. 



VIII. Eighth " " 



IX. Nintli 



1-17, 20, from Packard, DavAoYtment oi Limulus Polyphemus. Mem. Boston Soc. Nat. Hist., II., PI. III. -V., 1872. 



1. Spermatozoa, magnified about 400 diameters. 



2. Early form of ovarian egg, magnified 130 diameters. 



3. Embryo within the egg. m, mouth, a, inner egg-membrane, the "protoderra" or "amnion" of Packard, 



"chorion" of Dohrn. Outside the inner membrane is seen the outer egg-membrane, the "chorion "of 

 Packard, "exochorion" of Dohrn. The rudiments of the si.x anterior pairs of appendages have appeared. 

 The anterior pair of appendages of Limuliis, as shown by A. Milne-Edwards and Packard, are innervated 

 from the cesophageal commissure, and are probably homologous with the mandibles of Crustacea. Balfour, 

 moreover, has shown that in the spiders the anterior pair of appendages (chclicerse) in the embryo are 

 innervated from a post-oral ganglion, and are ei^uivalent to the mandibles of insects, rather than to the 

 antenn;e as commonly supposed. Around the edge of the oval germ is a thin ridge, destined to he the 

 lower edge of the carapace. 



4. The embryo in a later stage. Letters as before. 



5. Older stage seen from below. The seventh and eighth pairs of appendages, VII, VIII, have appeared. 



Above the lower margin of the carapace are seen the indications of the somites, the sutures extending up- 

 ward, but not reaching the dorsal side of the egg. The six anterior pairs of appendages have lengthened and 

 become bent upon themselves. 



6. Later stage of the embryo, viewed from the side. The body has now a decided ventral flexure. The ninth 



pair of appendages, IX, have made their appearance, the posterior division of the body has become 

 clearly differentiated from the anterior portion, and its somites well marked. The six anterior pairs of 

 append.ages have become jointed. I, liver. 



7. Rudimentary gills from an older individual. 



8. Terminal part of sixth pair of appendages. 



9. Dorsal view of the embryo just before hatching. Trilobitic stage. The egg-membrane, " amnion," (the outer, 



or ' ' chorion "of Packard, having been cast off before this period, ) is not represented in the figure. The egg 

 is now .13 in. in diameter. The enjbryo has already undergone its first moult within the egg. h, dorsal 

 vessel. c, compounil eye. o o I, ocellus. At an earlier jieriod than that represented in this figure the 

 ocelli are situate on the under side of the head, just in front of the mandibles. A little later they appear 

 on the front edge of the carapace. By the expansion and extension of this edge they are finally brought 

 to the upper side of the head, a little way fron; the front edge, as in the figure. 



10. Ventral view of the same stage, m t, metastoma or lower lip. n, nerve cord. 



11. Terminal portion of third pair of appendages. Same stage as the two preceding figures. 



12. Newly hatched young, viewed from in front and above. 



13. The same, viewed from behind and above. 



14. Dorsal view of newly hatched young. The segmentation of the posterior division of the body has become 



obscured. 



15. Ventral view of the same. 



16. Seventh pair of appendages of larva, which form the operculum of the adult. 



17. One of the eighth pair of appendages of larva, bearhig the gills. The two-jointed inner ramus is distinctly 



formed. 

 IS. Young at the time of hatching. From Dohrn, Unter.suchungen uber Ban und Entwickelung der Arthro- 

 poden. 12. Zur Embryologie und Morphologic des Limulus Pulyphcmus. Jenaische Zeitschr., VI., 

 Taf. XIV. fig. 4, 1871. 



19. Larva, from a sketch by A. Agassiz, made at Naushon Island, Mass., Dec. 19, 1864. The line on the right 



of the figure indicates the natural length of the larva. 



20. Larva after the first moult subsequent to hatching (about three weeks after hatching). It is now ^ in. 



long. The spine has acquired a considerable length. The arrows indicate the course of the circulation 

 as seen in the living larva, the feathered arrows denoting the arterial currents, the simple an'ow's the 

 course of the venous blood. The dendritic outline in the head is the liver sending two lobes backward 

 into the hinder part of the body alongside the dorsal vessel, whicli lies in the median line. The dorsal 

 vessel, is furnished with seven pairs of venous openings. Below the dorsal vessel, indicated by the fine 

 lines within it, the intestine is seen extending back toward the spine. 



