CHROMOSOME NUMBERS IN METAZOA 59 
There is however a great range of numbers among the different 
forms of animals. As is well known, there is only one chromo- 
some in the haploid groups of Ascaris megalocephala univalens; 
some species of Gordius (Nematode) also are reported as having 
only one chromosome in the reduced groups and Styelopsis (As- 
cidian) as having only one in the spermatid. Indeed, according 
to Moore, ’93, there is only one chromosome in the oogonia of 
Apus (Phyllopod crustacean). Animals having only two chromo- 
somes in the haploid groups are: Ascaris megalocephala bivalens, 
Cyclops viridis brevispinosus, Pediculopsis graminum (arachnid), 
Icerya purchasi (Homoptera), Tetrastemma vermiculus (Ne- 
mertean), Vortex viridis and Paravortex cardii (Rhabdocoels). 
At the other end of the series are: two species of Cambarus (Dec- 
apod), with 104 and 100 (reduced), Artemia (Phyllopod) with 
84, Cancer and Hippa (Decapods) with 60, Astacus (Decapod) 
with about 58 and Nyssia (Moth) with 56. The number occur- 
ring most frequently among the forms investigated is 12; other 
numbers occurring very frequently are 6, 7, 8, 9, 10, 11 and 16. 
There is also often a considerable range in number among dif- 
ferent forms belonging to the same class, e. g. Nematoidea (1-24 
reduced), Aphidae’ (3-20), Copepoda (2-17). The classes show- 
ing the greatest constancy are the Acrididae (Orthoptera) and 
the Urodeles (Amphibia). The Diptera and the Nematodes 
have, in general, low numbers whereas the Decapods and Lepidop- 
tera have high numbers. 
A chromosome is really a compound structure, carrying many 
characters or genes which are themselves the elements of he- 
redity. However genes may arise, it is conceivable that in some 
cases one or more new genes may be placed in a chromosome 
without disturbing its integrity, the number of chromosomes in 
related species thus remaining the same. On the other hand such 
additional genes may disturb the existing complex and cause the 
whole mass of genes to be entirely redistributed, thus causing a 
change in chromosome number in nearly related species. Should 
a certain group of genes be placed in one chromosome in one 
Species and in two in another, there would not be necessarily 
any difference in these two species. It would seem, however, 
