CHROMOSOME NUMBERS IN METAZOA 61 
number is 6, for the Echinoderms 18, for the Amphibia, the only 
class of Vertebrates satisfactory for generalizations, itis 12. For 
the Plathelminthes, the type number is 8, for the molluses 16, 
for the Annelids 16. As one might expect in a group with so many 
distinct subgroups, the Arthropods have several type numbers. 
For the Crustacea it is 8 (the Malacostraca have higher num- 
bers), for the Hemiptera 7, Orthoptera 12, Coleoptera 10, Diptera 
6, Lepidoptera 31. It is of interest that the type numbers in the 
enterocoelous series are all multiples of 6 or 6, whereas those of 
the teloblastic series (except the tracheates) are multiples of 8 
or 8. It is also of interest that the molluscs and Annelids which 
are so closely related have the same type number 16. The sub- 
groups which are degenerate or highly modified (e. g. Trematodes, 
Acanthocephala) usually do not have the type number of their 
groups. The data for the insects are the fullest and the most 
reliable and these offer the best study of changes in chromo- 
some numbers. 
The type number for the Hemiptera is 7 (haploid), including 
an XY pair or an X. Other numbers occur, all of which can be 
attributed to the fusion or splitting of chromosomes of the type 
group. The two Thyantas have been a puzzle, owing to their 
likeness in form and the wide divergence in chromosome number. 
Thyanta custator has a diploid number of 16, which would mean 
8 haploid including X or Y. Thyanta calceata has 28 in the 9 
diploid, 277 (owing to X being of two parts, Y of one), which 
would mean a haploid number of 149, 13. These numbers 
may be explained on the supposition that in Thyanta custator 
one chromosome of the type group has split in two, whereas in 
Thyanta calceata all of these have split except Y. Evidence of 
this splitting is given by X, which is of two parts, not always sep- 
arate in T. caleeata, each about the same size as Y and about 
half as large as the X in T. custator. This explanation may be 
expressed as follows: 
TYPE THYANTA CUSTATOR T. CALCEATA 
2 6+-X 6+1+X 6+6+2XxX 
of 6+Y Caine 6+6+Y 
