SOME PROBLEMS OF ANNELID MORPHOLOGY. $/ 



striking, nowhere less open to dispute. It is therefore 

 clear that from a comparative point of view a peculiar 

 interest attaches to these animals. It is nearly certain 

 that they are closely related to the ancestors of the 

 arthropods ; many zoologists regard them as closely 

 approaching the progenitors of the vertebrates. In any 

 case, no one who wishes to gain any insight into the 

 morphology of the higher segmented types can afford to 

 pass by the annelids, even though their remarkable 

 similarities to the higher types in organization and 

 development be regarded merely as analogies and not 

 as evidence of direct genetic connection. 



The importance of the annelids is heightened by 

 another remarkable fact. All annelids, in the course 

 of development, pass through a larval stage (Fig. 2) 

 known as the trochospJiere or trocJiophore, often disguised 

 but always present in some form. This larval type, 

 under many different modifications, occurs in many 

 other groups of invertebrates, though nowhere so clear 

 and typical as among the annelids. Its significance is 

 one of the most vexed questions of comparative mor- 

 phology, and opinion is at present nearly equally divided 

 between two opposing schools. According to one view 

 the trochophore is the embryological (or ontogenetic) 

 representative of an ancestral (phylogenetic) type, the 

 "Trochozoon," from which all forms passing through a 

 trochophore stage have actually been derived by evolu- 

 tion — just as a bird, for instance, is supposed to have 

 arisen from a fish-like ancestor because it passes through 

 a fish-like stage of development within the Q%^. If 

 this view be well founded, and if (observe the double 

 condition) the similarities between annelids and verte- 



