108 The Microscope. 



while in the young the uro-genital system exhibits metamerism 

 in a marked degree. The heart is but a specialized portion of 

 the aorta, which at first extends as a simple tube the whole 

 length of the body. Among the other forms which exhibit this 

 serial repetition of parts are most of the higher members of the 

 old, but heterogeneous group of worms, the brachiopods, the 

 arthropods (Crustacea and Insects), and to a less extent the 

 Mollusca. In this last group segmentation appears in only a 

 few instances, and in a limited number of organs. The chitons 

 have it best developed. 



To account for this serial repetition of parts, two theories 

 are prominent. The first to be mentioned is the older. As is 

 well known, reproduction by division is common among many 

 of the lower invertebrates. In some cases the body becomes 

 long, and then divides at or near the middle. Each half repro- 

 duces all the parts of the parent. The division is not instanta- 

 neous, but gradual. A constriction appears on the surface, and 

 gradually sinks in until at last a complete separation is effected, 

 Sometimes, before the first division is complete, a second one 

 sets in. and in turn a third. This is well illustrated in Micro- 

 stomum, as described by Graff. In Microstomum Catenula and 



jr. J H IS HE' 



Fig. 1. Self division in Microstomum (after Graff). I, II, III, IV, lines of suc- 

 cessive divisions; m, mouth of the whole animal; m 8 - 3 - 4 , mouths of the succes- 

 sive generations. 



allied forms the division is eventually completed, but we have 

 only to imagine the partially divided form to become perma- 

 nent and we have metameric segmentation. The tape- worms 

 need but be alluded to as an instance of this. 



The other hypothesis is more recent and more complicated, 

 though, on the whole, far more satisfactory. The germs of the 

 idea were first advanced by Arnold Lang, and afterward car- 

 ried out farther by Balfour, Wilson, and especially by Sedg- 

 wick. It has the merit of introducing homology throughout 

 almost the whole of the Metazoa. 



The ancestral form to which almost the whole animal king- 



