204 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



thus be seen that from the mouth to the one anus 

 there are 158 segments, and from the mouth to the 

 other, 144. Now, in the common earthworm there 

 are normally about 140 segments> so that not only 

 has there been increased activity in producing two 

 posteriors instead of one, but there has also been 

 an increase in the length of each of the two parts. 



In order to attempt an explanation of such a 

 monstrosity, it is necessary to look at the condition 

 in the early stages of development. Balfour, in his 

 Comparative Embryology, vol. i., p. 266, as the result 

 of Kowalevsky's investigations on Lumbricus agricola, 

 says, referring to the formation of the gastrula from 

 the flattened blastosphere : "An invagination takes- 

 place, in the course of which the hypoblast becomes 

 enclosed in the epiblast and a somewhat cylindrical 

 two-layered gastrula is formed. The opening in this 

 gastrula at first extends over the whole of what 

 becomes the ventral surface of the future worm, but 

 gradually narrows to a small pore — the permanent 

 mouth — near the end." The gastrula elongates, an 

 anus is formed, and after some further development 

 we get the perfect worm. 



Whilst this is the course of the early development 

 of Lumbricus agricola, Kleinenberg* has shown that 

 a different and very peculiar state of affairs exist* 

 in Lumbricus trapezoides, Duges, which throws a 

 considerable amount of light on double monsters. 

 He has shown that in this worm two complete 

 individuals are normally developed from one ovum.. 

 xlt a very early period, he states, it may be observed 

 that the blastosphere is composed of two more 

 or less distinct hemispheres. On the ventral surface 

 a transverse furrow is formed, " the endoderm 

 becomes invaginated, beginning at the lateral 

 margins of the furrow in both the hemispheres,, 

 which are thus transformed into sacs with double 

 walls. . . . Each of the lateral cavities will 

 form the digestive cavity of an individual, their 



* "The Development of the Earthworm, Lumbricus trapezoids, Duges." Kleinenberg, 

 Quart. Journ. Micro. Sc., vol. xix. (187V»), p. 206. 



