LIMULUS. 301 



The dermal bone tissue varies considerably in extent in different individuals. 

 It is not visible in specimens less than eight inches long and probably does not 

 make its appearance till the animal is full grown. Even when it is largely devel- 

 oped, the characteristic lacunas in the chitenous bars may be absent. This 

 shows, I believe, that the lacunae are not fully developed till long after maturity 

 is reached, for it was in the very oldest individuals, with much scarred and worn 

 armor, that I found them best developed. 



It is difficult to determine how the nuclei get into the lacunae. They appear 

 to migrate into them from the surface epithelium through the canaliculi. The 

 following observations lend some support to this view 7 . In the region of the ol- 

 factory organ, the chiten is soft and flexible and the bone cells are absent. When 

 stained with the ordinary nuclear stains, the whole thickness of the wall is seen to 

 be filled with minute sharply stained, nuclear-like bodies of uniform size, arranged 

 with considerable regularity in the pore canals. (Fig. 207, B.) 



In sections of the adult oesophagus (Fig. 207, .4 ), the chiten is seen to be filled 

 with bodies that also take a nuclear stain. Here they are of varying sizes; some 

 are minute dots, others much larger, and spindle-shaped, and arranged in rows that 

 follow- the undulations of the lamellae. The rows of spindle-shaped, or bead- 

 like bodies are often united by delicate threads, as though they had multiplied 

 by division and were still imperfectly separated from each other. 



It is possible that some of these bodies are bacteria, or the spores of a parasitic 

 fungus, or the products of some degenerative process. That is a point upon which 

 I have not been able to satisfy myself, one way or the other. It is certain that a 

 species of fungus, Macrocystis, does grow in the chiten of Limulus. Fragments 

 of the skin, stained with methylene blue, often show the deeply stained hyphae 

 ramifying in all directions through the substance of the chitenous covering of the 

 gills, or in the chiten surrounding the olfactory organ. They appear to dissolve 

 out channels in the chiten, which are then completely filled by the growing 

 hyphae. The characteristic spores of Macrocystis have been found in abundance 

 on the outer surface of the chiten in the olfactory region of dried shells. There is, 

 however, no suggestion of any connection between this fungus and the nuclear 

 bodies just described. 



The continuous external armor of arthropods must be shed at regular inter- 

 vals, to make room for growth, and the shedding of the old shell is always a diffi- 

 cult and dangerous process. In Limulus we see the beginning of a new type of 

 exoskeleton, one that is subdermal and discontinuous, that need not be, and never 

 is cast off after it is once formed. Indeed, Limulus could no more shed its dermal 

 bones than a vertebrate could shed its cartilage cranium or its vertebral column. 

 Limulus has therefore solved the problem for arthropods, of getting rid of an 

 impractical external covering, a covering which has become too cumbersome 

 and too impermeable for physiological purposes, which prohibits growth if 



