DOUBLE MONSTROSITY— CLASSIFICATION 



0. CLASSIFICATION. 



In Teratology, the terms (1) anadidym/us, (2) katadidymus, (3) anakatadidymus, and (4) meso- 

 didymus are accepted as indicating respectively (1) doubling at the anterior end, (2) doubling at the 

 posterior end, (3) doubling at both anterior and posterior ends, and (4) doubling in the middle region 

 of the body. As regards fishes, it should be noted that the first (anadidymus) is much the most 

 important ; the third is practically confined to examples of union by the yolk-sac only ; while the 

 second and fourth, so far as seems to be known at present, do not provide examples of com- 

 plete duplicity (p. 25). 



"We owe the first three terms (anadidymus, katadidymus, and anakatadidymus) to Forster 

 (Missbildungm des Mrnschen, Jena, 1861, pp. 22, 29, 34), but it is to be noted that this author 

 assigned to the first two of them a meaning exactly the converse of that in which they are now 

 employed, having apparently applied the initial preposition to indicate the region of union instead of 

 the region of separation. His anadidymus is thus the katadidymus of later authors. The fourth 

 term, mesodidymus, we owe to Oellacher (176), but for reasons that will appear afterwards (p. 25) 

 this term (as well as katadidymus) may with advantage be replaced by hemididymus, the substitute 

 suggested by Eauber (200 79) and adopted by Kopsch and others. 



Of the groups above named, only the anadidymi, by reason of their number and variety, require 

 detailed subdivision. The various schemes proposed need not be given here. Most authors dealing 

 with the subject have adopted simpler or more elaborate methods of classification depending on the 

 level at which union takes place. For external and descriptive purposes the scheme given by 

 Windle (272) will be found convenient and inclusive. His classes are as follows: 



1. Head union, three eyes present, all of the same size. 



2. Do. three eyes present, the median one being larger than the others. 



3. Do. four eyes present. 



4. Two quite separate heads. 



5. Fission extending to the pectoral region. 



G. Duplicity extending to the posterior border of the yolk-sac, the caudal extremity being 

 quite single. 



7. Duplicity extending a short distance behind the posterior border of the yolk-sac, the caudal 



extremity being quite single. 



8. Duplicity extending to the posterior border of the yolk-sac, the caudal extremity not, 



however, being perfectly single. 



9. Union by the caudal extremities alone. 



1 0. Union by the ventral aspects at the site of attachment of the yolk-sac. 



1 1 . Parasites, one member becoming an appendage to the other. 



A less convenient, though logically sound system, is proposed by Schmitt (216), who would 

 divide double monsters in fishes according to the manner in which they come together at the region 

 of union. He thus distinguishes : 



(1) Union by the yolk-sac only. (2) Simple ventral union. (3) Union principally ventral, 

 but partly lateral. (4) Union half lateral and half ventral. (5) Union principal] v 

 lateral, but partly ventral. (6) Simple lateral union. (7) Union so intimate that 

 externally the duplicity is concealed. 



The classification which will be adopted in this work is given below. It has reference alike to 

 external features and to important points of internal structure. 



Class I. Union in head region, the twin brains uniting at the optic lobes. 1 

 Class II. Union in head region, the twin brains uniting at the medulla oblongata. 



'Regarding the possible need for a Class to come in advance of my Class I., as showing a still less marked degree 

 of anterior duplicity, see the second last paragraph under Reduction and Parasitism (p. 59). 



