532 E. KAY LANKKSTEH. 



metameres or somites produced or is definite. Animals in the 

 first case we call anomomeristic ; those in the second case, 

 nomomeristic. The nomoineristic condition is a higher de- 

 velopment, a specialisation, of the anomomeristic condition. 



The Second Law, or generalisation, as to metamerism 

 which must be noted is that the meres or somites (excepting 

 the first with its prostomium) may be all practically alike, or 

 may differ from one another greatly by modification of the 

 various constituent parts of the mere or somite. Metamerised 

 animals are either homceomeric or heteromeric. The reference 

 to the variation in the form of the essential parts contained 

 in a "metamere " or " somite " introduces us to the necessity 

 of a general term for these constituent or subordinate parts; 

 they may be called " meromes" {fxlpog). The meromes pre- 

 sent in a metamere or somite differ in different annulate or 

 segmented animals according to the general organisation of 

 the group to which the animal belongs. As a matter of con- 

 venience we distinguish in the Arthropod as meromes, first, 

 the tegumentary chitinised plates called terga, placed on the 

 dorsal aspect of the somites ; second, the similar sternal plates. 

 In Ch^etopods we should take next to these the masses of 

 circular and longitudinal muscular fibres of the body-wall and 

 the dorso-ventral muscles. The latter form the third sort 

 of merome present in the Arthropods. The foui'th kind of 

 merome is constituted by the parapodia or appendages ; the 

 fifth by the coelomic pouches and their ducts and external 

 apertures (coelomo-ducts), Avhether renal or genital. The 

 sixth by the blood-vessels of the somite; the seventh by the 

 bit of alimentary tract which traverses it ; and the eighth by 

 theneuromere (nerve-ganglion pair, commissures, connectives, 

 and nerve branches). 



It becomes apparent from this enumeration that there are 

 a good many important elements or " meromes " in an Arthro- 

 pod metamere or somite which can become the subject of 

 heteromerism, or, to use a more apt word, of " heterosis." It 

 is all the more necessary to insist upon this, inasmuch as there 

 is a tendency in the discussion of the segmentation of the 



