BY K. J. GODDAUD. 63 



Structures, such as the anterior aiil posterior suckers were 

 -developed, and in the generation of these were concerned the 

 segments at the anterior and posterior extremities. In this con- 

 nection took place the fusion of the ganglia of the originnlly 

 distinct segments at these extremities. The traces of annulation 

 sometimes visible on the posterior suckers of members of the 

 Ilirudinea in general, and sometimes on the capula of the 

 IchthyohJelUdoe, support this view. Later came the necessity for 

 the extension of the somite in order to enable of an extension of 

 the body, and this represents tlie direct reason behind the 

 annulation of the leech-body, inasmuch ts, apparently, the somite 

 was incapable of giving rise to another distinct somite, or in 

 other words there lias been no increase in the number of ganglia, 

 although the nervous system has given rise to special branches 

 for the innervation of the newly acquired annuli which are so 

 supplied quite separately from the original primitive sensory 

 annulus, which represented, in fact, a sensory unit. This extension 

 of the sonite has bsen CDUcerned chief!}' in connection with the 

 greater part of the Ijody which may be termed more or less 

 "central." We tind the intermediate stages in the passage from 

 the uniannulate to the midtiannidate stage represented in proper 

 serial order, passing from either extremity to that portion of the 

 body in which the complete annulation characteristic of the genus 

 is found. In these intermediate somites we find the key to the 

 order of development of the annuli, nnd this order, although very 

 different in each of many genera, is constant in the species of 

 any one genus. It may not be out of place to remark here, tliat 

 we find in such forms as Ji ranchiohdella and B/e/lodrilas a 

 representative very closely allied to the hypothetical primitive 

 ancestor of the Hirudluea at the time of its leaving the mi,iii stem, 

 or very soon after. These organisms, although certainly not 

 members of the Hlrudinea, may represent in themselves, and 

 probably do, examples of homoplasy, in that they have developed 

 suckers; still the}' serve at the same time as examples of what 

 has been said above. 



