28o 



VARIATION AND MONSTROSITIES. 



which constitute the very foundation of arthropod morphology, are the varying 

 results of apical growth, locally checked by median fusion and degeneration. 



C. Acephalic and Acaudal Embryos. — A common abnormality consists in 

 the extensive reduction of the anterior or posterior metameres, or of both, leaving 

 only the middle portion of the trunk intact. It is not clear whether this result is 

 brought about by median fusion or not. 



It is a common thing to find embryos without the cephalic lobes; or without 

 cephalic lobes and the first three pairs of thoracic appendages; or the abdomen 



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Fig. i86. — Limulus embryos in advanced stages of degeneration. A , Stage G. The anterior half of the thorax 

 is absent and the isolated procephalic lobes are reduced to a thin depressed plate, with an underlying mass of yolk 

 cells. Cam. X 30. B.StageG. Anterior half of thorax absent, and the greatly reduced cephalic lobes covered by 

 a thick fold of ectoderm. Cam. X30. C, Stage G. The abdomen and the anterior half of the thorax absent. 

 Last three thoracic appendages on the left, reduced to shallow pits; cam. X 30. D, Germinal disc, with three un- 

 paired thoracic appendages; cam. X30. E, Remnants of an embryo, probably in stage G; only the tip of the ab- 

 domen, or the unpaired remnants of one of the more posterior pairs of appendages, projects above the surface of 

 the faint embryonic area; cam. X60. F~K, Similar embryos in more advanced stages of degeneration. 



alone may be present, without the head and thorax. The most common form con- 

 sists of little more than the three posterior thoracic metameres. Such embryos 

 may have a deceptive resemblance to a crustacean nauplius. (Fig. i86, B.) 



Such cases as these, and the hour-glass type, show that certain groups of 

 metameres have an organic unity that enables them to survive, at least for a certain 

 period, independently of other parts of the body. It suggests the phenomena of 

 transverse division in the annelids; the remarkable process in cirripeds (Sacculina) 

 by which the abdomen is cast off, leaving only the thorax to complete the life cycle; 



