DEGENERATION. 277 



Asymmetery is a common abnormality in Limulus and is expressed in a variety 

 of ways. It is most easily observed in the appendages, but in most cases, appar- 

 ently, changes in them are accompanied by similar ones in the corresponding half 

 neuromeres, the somites, and sense organs. 



A. Multiple Organs. — Asymmetry due to the presence of extra organs on 

 one side is very rare. A good illustration is seen in Fig. 184, E., where the first 

 thoracic half metamere has divided twice, giving rise to two half neuromeres, 

 two left lateral eyes, and four imperfectly divided left chelicerae. 



B. Defective Organs. — Asymmetry due to the absence, or reduction of ap- 

 pendages is very common, and apparently occurs as frequently on the right side 

 as on the left, and as frequently in the thorax as the abdomen. (Fig. 184.) But 

 half embryos like that in Fig. 184, C, are very rare. Here all the organs on the 

 right side, except what appears to be the sixth leg, are absent; the left side appears 

 perfectly normal except for its curvature toward the right. 



Asymmetry similar to that seen in Limulus has become a fixed character in 

 certain groups of arthropods, e.g. hermit crabs and bopyridae. The radial symmetry 

 of the echinoderms was brought about by the loss of one side of the body probably 

 in some arthropod-like ancestor. The remaining side taking the form of a closed 

 ring established a successful organic union and laid the foundations for a 

 new type of radiate structure and a new class of animals. See Chapter 

 XXIII, p. 421. 



III. Degeneration. 



A. Median Fusion and Antero-posterior Degeneration. — This remark- 

 able phenomenon is so common, and has been observed in so many different stages, 

 that there can be no doubt of the manner in which it takes place. The process 

 starts in the anterior metameres and is taken up by the following ones in numerical 

 order. In the typical cases, each organ unites with its fellow of the opposite side 

 to form an unpaired organ, which then disappears. Those nearest the median 

 line unite first, and after they degenerate those lateral to them unite and degener- 

 ate in the order of their position, till the whole of the metamere has disappeared. 

 As the process in one metamere is always a step in advance of that in the next 

 posterior metamere, A-shaped embryos are produced showing various stages in 

 the progress of degeneration. The successive steps are most clearly shown by 

 the appendages, the dorsal organs, and the nerve cords. The other paired organs 

 probably fuse and degenerate in the same manner, but their history is not so easily 

 followed. Toward the close of degeneration we may find, at what was the pos- 

 terior end of the embryo, either a median row of unpaired organs, or an exhausted 

 mass of cells, and finally they may in their turn disappear, leaving no trace of the 

 embryo behind. 



