A third group of double autositic monsters are the Terata Anakatadidyma, which are divided above and below, but joined from the navel to the head. There are three genera. The first, the Prosopothoracopagus, is joined at the upper abdomen, the chest, and the faces; the spinal columns are separate. The faces are imperfect, the jaws are united; there is a broad neck with one oesophagus, and there is one stomach and one duodenum. This is a rare form, and it can not exist out of the uterus.
A second genus, the Thoracopagus, has a thorax in common, and the inner legs may be united. It is, as a rule, still-born.
The next genus is the Omphalopagus, in which the twins are joined from the navel to the bottom of the chest. This double monster has the slightest union of all, and it is very rare. The Siamese Twins were omphalopagi. They quarrelled; one became a drunkard and the other remained temperate. They married two women, and Chang had ten children, and Eng twelve. Chang died while Eng was asleep, and the latter died two hours after he had waked and learned of his brother's death.
There is a genus, the Rachipagus, the examples of which are joined behind like the class Terata Anakatadidyma that are joined in front.
Four known attempts have been made to separate double monsters surgically, but all failed owing to crude surgery; modern methods might be successful in some cases.
The second order of double monsters comprises the parasitic class. There are three genera of these terata, with five species and seventeen varieties. The chief of these only will be mentioned. The Heterotypus is a parasitic child which hangs from the abdominal wall of the principal subject. Varieties of this species are the Heteropagus, which is a parasite with head and arms; the Heterodelphus, which has no head; the Heterodymus, which has a head, neck, and thorax. The Heteralitis is a second species, in which the parasite is inserted at a distance from the navel of the autosite. The Epicomus is the only example, and it consists of a parasitic supernumerary head. The Polypnathus is a parasite attached to the jaw of the autosite. When fastened to the upper jaw, it is an Epignathus; at the lower jaw it is an Hypognathns. Another group is made up of terata having parasitic legs which are attached to different parts of the autosite,—to the pelvis, the head, the abdomen, and so on. Finally, there is the Endocyma, which is a parasite enclosed within the body of an autosite.
Parasites are nourished through the blood supply of the autosite, and the parasites usually are incapable of motion. The autosite can feel when the parasite is touched, and in some cases the autosite can localise the touch. In India, in 1783, a child was born which had a supernumerary head attached to the autositic head, crown to crown; it lived four years. The parasite's eyes were always partly open, but they appeared to be incapable of intelligent vision. They contracted under strong light, and when the autosite was suddenly awakened both sets of eyes moved.
Gould and Pyle (Anomalies and Curiosities of Medicine) give an account of an Italian boy, aged eight years, who had a small parasitic head protruding from near the left third rib. Sensibility was common. Each of the heads received baptism (one was called John and the other Matthew), and there was question as to whether extreme unction should be administered to the parasitic head. A similar case occurred in [{86}] England in 1880 (British Med. Journal), and the parasitic head could be pinched without attracting the attention of the autosite.
Teratologists now exclude Dermoid Cysts from the lists of terata. The hair, teeth, and particles of bone found in these cysts are looked upon as the development of abnormal ectodermic and endodermic cells, rather than as evidence of a separate personality.