236 INVEETEBKATA CHAP. 



scorpion that even an approximately complete account of the life- 

 history has been elucidated. 



If we turn to Kishinouye's account of the development of Limulus 

 (1893) we find that for the earliest period of development only an 

 incomplete series of stages was at his disposal, and that, if his con- 

 clusions are correct, Limulus differs from the spider inasmuch as 

 the nuclei which represent the endoderm are budded into the yolk 

 before the keel of the primitive thickening is formed. Kingsley 

 (1892-1893) describes the cells forming the blastoderm as dividing 

 tangentially into a small clear moiety which remains at the surface, 

 and a large inner half, full of yolk grains, which is endodermal and 

 wanders off into the yolk. 



The primitive thickening, when it appears, is said to give rise 

 only to mesoderm. A similar statement, as we shall see later, has 

 been made for insect development, but it seems clearly to be erroneous 

 for insects, and may prove to be so also for Limulus. We must bear 

 in mind that the epithelium of the gut of Limulus is formed very 

 late, long after the larva is hatched, and that it appears first of 

 all in the region of the abdomen. Further, no coelomic sacs at all 

 appear in the bases of the appendages of the prosoma, except in the 

 fifth and sixth, and these form the coxal gland. It follows that the 

 coxal glands of Limulus and the spider are not strictly homologous 

 with one another, but that both are remnants of a once complete 

 series of metamerically arranged excretory organs. 



As there is no anterior aorta in Limulus we find that the coelomic 

 sacs of the cephalic lobes do not meet each other, but that a vascular 

 circumoesophageal collar is formed by the shrinkage of these sacs 

 from the sides of the oesophagus. The heart is formed in exactly the 

 same way as in the spider. According to Kingsley. after giving rise 

 to the heart-wall the coelomic sacs fuse longitudinally with their 

 predecessors and successors, and so two persistent tubes are formed 

 which, he thinks, give rise to the genital organs, as they do in 

 PerijKitus (see p. 1*75). 



The dorsal and ventral surfaces develop in even proportion with 

 one another, so that there is no process of reversion. The last 

 appendage of the prosoma, in the adult, has an outer branch called 

 the epipodite or flabellum, which is probably the remnant of an 

 exopodite, such as forms the major part of the limb in the case of 

 the appendages of the abdomen. In the embryo all the appendages 

 of the prosoma except the first, develop the beginnings of this exopodite, 

 but it only persists in the sixth and last. (Fig. 18*7, flab 1 ' 5 .} 



The central eyes of Limulus are inverted like the posterior 

 central eyes of the spider, and originate from a corresponding part 

 of the ectoderm ; in front of the brain rudiment on the ventral plate, 

 measuring along the ventral surface, behind if we measure on the 

 dorsal surface. The lateral eyes of Limulus originate also as a single 

 undivided pit on each side, as such pits begin in the spider ; but 

 though they develop many ommatidia, formed by the grouping of some 



