OF ARTS AND SCIENCES. 88 



rpmaitiing three somites have lost 3 rings each, making a loss of 11 

 rings. This syncopation of 28 rings, at the two ends of the body, and 

 at least as many more in the acetabulum, is not to be regarded as an 

 actual loss ; these rings have been sacrificed rather in tlie interest of 

 the rings retained. The loss at the anterior end is correlated vs^ith a 

 higher development of the sense-organs; at the posterior end, with a 

 greater development of muscles. It is interesting to note that Natural 

 Selection has played a part in deciding the fate of these rings ; for the 

 papillate rings have been preserved, while the non-papillate rings have 

 been in part or wliolly suppressed. 



Is this suppression of rings still going on ? or has it reached a limit ? 

 A closer examination of the rings in the terminal somites will show 

 that the process of abbreviation is still advancing ; and a comparison 

 of different genera proves that its progress has not been evervwhere 

 equally rapid. The suppression of rings takes place by consolidation, 

 two successive rings coalescing gradually. The papillate ring may 

 unite with, or absorb, either the preceding or the following ring. In 

 the Medicinal Leech, the oth ring, which bears the fourth pair of ej'es, is 

 now in process of uniting with the Gth ; while the 8th is absorbing the 

 7th. The evidence that these two rings are being swallowed up is seen, 

 first of all, in the rings themselves ; and secondly, in the different condi- 

 tions which they present in different species. In Ilirudo and several 

 allied genera, the Gth and 7th rings are relatively narrow ; and the 

 grooves separating them from the 5th and 8th rings are obliterated on 

 the ventral side, so that here the four rings ai)pear as two. On the 

 dorsal side they are still distinct, but not so deeply marked off from 

 the oth and 8th rings as from each other. The same process of con- 

 solidation is seen in IIa;mopis, Aulostoma, and Hasmadipsa, but in 

 slio-htly different stashes. In Macrobdeila all four rino;s are distinct 

 on both sides; but the consolidation has ali-eady begun, as the grooves 

 separating the 5th from the Gth, and the 7lh from the 8th. are not so 

 deep as the groove between the Gth and 7th, or as that between any 

 two of the succeeding rings. 



If centripetal abbreviation be the law of development, we should ex- 

 pect the 4th ring to disappear before the Gth and 7th. This course of 

 events lias already been realized in the land leeches (Fig. 2), in none 

 of which is there a ring intervening between the third and fourth pairs 

 of eyes. 



Two non-papillate rings may also unite : an instance is seen in the 

 23d somite of ]Macrobdel la, where the 2d and 3d rings (95th in Fig. 4) 

 are wellnigh consolidated. 



