INVAGINATED APPENDAGES. 275 



expected in such an ancient type as Limulus, although there is no reason to believe 

 that the range of variation in Limulus is exceptional. 



Our observations show that there is a very small range in the kind of variation, 

 for it is of a plus or minus nature in practically all cases. They also show that 

 the normal forms follow a set course; extreme divergence from it leads to complete 

 extinction by a process of degeneration that is the reverse of the process of genera- 

 tion, the various organs disappearing in the same order in which they made their 

 appearance. 



I. INVAGINATED APPENDAGES. 



This common abnormality consists in the transformation of the usual 

 outgrowing appendages, in whole or in part, into ingrowing pockets. 



The infolding may take place in the comparatively late stages of its develop- 

 ment, only the tip of a well developed leg being infolded (Fig. 183, C.) ; or the entire 



FIG. 183. Three Limulus embryos in about the same stage of development (stage G-H) and drawn to the same 

 scale. They show the variations in the size of the appendages, and of the embryos as a whole; also the varying 

 extent to which the appendages are infolded. A, All the thoracic appendages are invaginated, except the first and 

 last pairs; B, the fourth pair completely invaginated; C, the third pair completely invaginated, and the tip of 

 the fourth appendage of the right side. Camera. 



leg may, from the beginning of its development, grow inward instead of outward) 

 forming a deep pocket opening outwardly by a transverse slit. (Fig. 183, B., 



Any thoracic appendage, except possibly the first, may be invaginated. In a 

 given embyro the infolding may affect one appendage, or a pair, or several append- 

 ages on one, or on both sides. Infolded legs are found in otherwise normal em- 

 bryos, or in those presenting other abnormalities; never in the adult. 



The conditions under which they occur clearly show that they are in all 

 probability produced by some special condition within the appendage itself, 

 not by local pressure, or by any other external cause. 



The frequent occurrence of invaginated appendages in Limulus embryos is 

 an important fact. We may infer that similar " abnormalities " occurred in other 

 arachnids and for some unknown reason became "fixed" or " normal," giving 

 rise to the infolded abdominal appendages which form the basis of the lung 

 books. In the tunicates, in Balanoglossus, and in vertebrates, similar respiratory 



