70 WILLIAM BATBSON. 



have been raised and have been variously replied to, and in 

 this condition the matter rests. By those who support it, it is 

 assumed that the common feature of segmentation is so binding 

 and unique a property as to suffice to link together groups 

 whose morphology is otherwise widely different. 



In the following pages it is proposed to examine the pro- 

 priety of employing the character of metameric segmentation 

 as one of first importance in forming a phylogeny of this kind. 

 And before referring to the evidence derived from the fact that 

 the three characteristic features of Chordata are found in 

 Tunicata and Enteropneusta, which are unsegmented forms, it 

 will be best first to discuss the meaning of the phenomenon — 

 ''segmentation" — for if resolved into its elements it will be 

 found to be by no means a peculiar feature of a few groups, 

 but rather the full expression of a tendency which is almost 

 universally present. 



The term "metaraeric segmentation" has been used to 

 describe several anatomical features, which reach their highest 

 development in the Annelids, the Arthropods, and the Verte- 

 brata. If an attempt be made to reduce this expression to its 

 simplest terms it appears to mean, in the first place, that cer- 

 tain organs of the body are serially repeated from before back- 

 wards, and in the second place that, in the case of the 

 Vertebrates and Annelids at all events, the body cavity is at 

 some period of life divided into a series of compartments, each 

 of which is closed off from its neighbours. But when a more 

 precise account of this phenomenon is required, and when it 

 becomes necessary to particularise as to which of the various 

 organs of the body is thus repeated, difficulty at once arises 

 from the fact that this repetition is irregular, and even within 

 narrow limits may vary considerably. In the case of many of 

 the errant Polychaets all the mesoblastic organs, together with 

 certain apparently serially homologous parts of the nervous and 

 digestive systems may recur for a seemingly indefinite number 

 of times in one individual, or even the whole animal may be 

 repeated in a chain, thus giving the highest expression to 

 the phenomenon. On the other hand, as in Lumbricus, 



