538 E. RAY LANKESTER. 



A^ertebrata do conform to the same law as the Arthropods. 

 Intercalation of meromes — branchial, vertebral, and dermal 

 (fin-supports) — seems to have taken place in Vertebrata in 

 the fishes, while in higher groups intercalation of vertebrfe in 

 large series has been accepted as the only possible explana- 

 tion of the structural facts established by the comparison of 

 allied groups. The elucidation of this matter forms a very 

 important part of the work lying to the hand of the investi- 

 gator of vertebrate anatomy, and it is possible that the 

 application of Goodrich's law (the seventh of our list) may 

 throw new light on the matter. 



In regard to the diminution in the number of somites in the 

 course of the historical development of those various groups 

 of metamerised animals, which have undoubtedly sprung 

 from ancestors with more numerous somites than they them- 

 selves possess, it appears that we may formulate the following 

 laws as the tenth, eleventh, twelfth, and thirteenth laws of 

 metamerism. 



The Tenth Law is that individual somites tend to atrophy 

 and finally disappear as distinct structures, most readily at 

 the antei'ior and the posterior ends of the series constituting 

 an animal body. This is very generally exhibited in the head 

 of Arthropoda, where, however, the operation of the law is 

 largely modified by fusion (see below). With regard to the 

 posterior end of the body, the atrophy of segments does not, 

 as a rule, affect the telson itself so much as the somites in 

 front of it and its power of producing new somites. Some- 

 times, however, the telson is very minute and non-chitinised 

 (Hexapoda). 



The Eleventh Law may be stated thus : — Any somite in 

 the series which is the anterior or postei-ior somite of a tagma 

 may become atrophied, reduced in size, or partially aborted 

 by the suppression of some of its meromes ; and finally, such 

 a somite may disappear and leave no obvious trace in the 

 adult structure of its presence in ancestral forms. This is 

 called the excalation of a somite. Frequently, however, such 

 "excalated" somites are obvious in the embryo or leave some 



