556 E. EAY LANKESTER. 



by numerical position of the segments) — in one case antenni- 

 form, in another chelate, in another pediform, and in another 

 reduced to a mere stump or absent altogether. Very 

 probably the power Avhich the appendage of a given segment 

 has of assuming the perfected form and proportions previously 

 attained by the appendage of another segment must be 

 classed as an instance of " homoeosis," not only where such a 

 change is obviously due to abnormal development or injury, 

 but also where it constitutes a difference permanently 

 established between allied orders or smaller groups, or between 

 the two sexes. 



The most extreme disguise assumed by the Arthropod 

 parapodium or appendage is that of becoming a mere stalk 

 supporting an eye, a fact which did not obtain general 

 credence until the expei'iments of Herbst, in 1895, who found, 

 on cutting- off the eye-stalk of Palasmon, that a jointed 

 antenna-like appendage was regenerated in its place. Since 

 the eye-stalks of Podothalmate Crustacea represent append- 

 ages, we are forced to the conclusion that the sessile eyes of 

 other Crustacea, and of other Arthropoda generally, indicate 

 the position of appendages which have atrophied.^ 



From what has been said it is apparent that we canuot, in 

 attempting to discover the affinities and divergences of the 

 various forms of Arthropoda, attach a vei*y high phylogenetic 

 value to the coincidence or divergence in form of the append- 

 ages belonging to the somites compared with one another. 



The principal forms assumed by the Arthropod parapodium 

 and its rami may be thus enumerated : 



(1) Axial corm well developed, unsegmented or Avith two 

 to four segments; lateral endites and exites (rami) numerous 

 and of various lengths (certain limbs of lower Crustacea). 



(2) Corm, Avith short, unsegmented rami, forming a flat- 



' ir. Milne-Edwards, wlio was followed by Huxley, long ago forniulalcd the 

 conclusion that the eye-stalks of Crustacea are modified a])])eMdages, basing 

 his argument on a specimen of Palinurus (figured in Bateson's book) (1), in 

 which the eye-stalk of one side is replaced by an anteniiifonii palp, llofer (6) 

 in 1894 described a similar case in Astacus. 



