66 ETHEL BROWNE HARVEY 
has been described in some Myriapods, most Arachnids (Araneida) 
and a few Copepods, but in no other Crustacea and not in Peripa- 
tus. Sex chromosomes have not been described in any Annelids, 
Coelenterates, Nemertines, Porifera, Rotifera, Protochordates or 
fishes and for only one Plathelminth. A few cases of their 
occurrence have been reported in the Echinoderms, molluses, Am- 
phibia, Birds, Reptilesand mammals. In the Nematodes they are 
of frequent occurrence. In allcases it isthe o& which is hetero- 
zygous except in the Lepidoptera and birds and the genetic evi- 
dence agrees with the cytological. Although X and XY are typ- 
ically single elements, the X consists In some cases of two or 
more elements closely or loosely associated. An unpaired X of 
two elements has been described for Syromastes, two species of 
Phylloxera, Leptinotarsa, Agalena (spider), Hyalocylis (mollusc), 
Schistosomum (Trematode), Rhabditis, fowl, and man and pig: 
an X of two elements accompanied by a single Y for Thyanta 
calceata, Gryllotalpa borealis, Cincindella and some of the 
Reduvioids. In other Reduvioids, Galgalus, Notonecta indica 
and Ascaris incurva, the X of an XY pair consists of three 
or more parts—of 8 in the last named, and in Ascaris lumbri- 
coides and A. canis, an unpaired X consists of five and six ele- 
ments respectively. A Y consisting of several elements has 
been described for Phragmatobia, and a Y of two elements in 
one testis of Odontota (Coleoptera). In a few cases the sex 
chromosomes are attached to another chromosome during part 
at least of their history: Ascaris megalocephala, Leptynia, Dix- 
ippus, Hesperotettix, Mermeria bivittata, Anopheles punctipen- 
nis and Necturus. 
Another set of heterochromosomes are the supernumerary 
chromosomes, which typically accompany sex chromosomes and 
divide in only one division, but are distributed irrespective of 
X and Y; they are constant in number in an individual, but differ 
in different individuals of the same species. They have been 
found in Metapodius, Euschistus variolarius, Banasa calva, 
Diabrotica, Ceuthophilus, Drosophila ornatipennis, Circotettix, 
Trimerotropis, Hesperotetitix viridis, Tettigidea parvipennis, some 
spiders and Necturus. 
