536 E. liAY LANKRSTER. 



by the names cervical, thoracic, lumbar, and sacral, are trans- 

 lated (accompanied by terminal increase or decrease in the 

 total number of somites) so as to occupy differing numerical 

 positions in closely allied forms (cf. the varying number of 

 cervical somites in allied reptiles and birds). 



What, in this rapid enumeration, Ave will venture to call 

 tlie Eighth Law of metamerism is the law of hoinoeosis, as it 

 is termed by Bateson (1). Homososis is the making of a 

 merome into the likeness of one belonging to another meta- 

 mere, and is the opposite of the process of 'Mieterosis" — 

 already mentioned. We cite this law here because the result 

 of its operation is to simulate the occurrence of dislocation 

 of meroraes, and has to be carefully distinguished from that 

 process. A merome can and does, in individual cases of 

 abnormality, assume the form and character of the corre- 

 sponding merome of a distant somite. Thus the antenna of 

 an insect has been found to be replaced by a perfectly well- 

 formed walking leg. After destruction of the eye-stalk of a 

 shrimp a new growth appears, having the form of an antenna. 

 Other cases are frequent in Crustacea as individual abnor- 

 malities. They prove the existence in the mechanism of 

 metamerised animals of structural conditions which are 

 capable of giving these results. What those structural con- 

 ditions are is a matter for separate inquiry, which we cannot 

 even touch here. It is not improbable that homoeosis of 

 distant meromes may have given rise to permanent structural 

 changes characteristic of whole groups of Arthropoda, sup- 

 posing the abnormality once established to be favoured by 

 natural selection. Possibly the chelate condition of the prte- 

 oral appendages of Arachnida may be due to homoeosis trans- 

 ferring the chelate form of post-oral limbs to what were 

 previously antenniform rami. 



We now come to the question of the production of new 

 somites or the addition of new somites to the series, and the 

 converse problem of the suppression of somites, whole or 

 partial. We state as the Ninth Law of metamerism "that 

 new somites or nietamcres are added to a chain consisting of 



