SOME PROBLEMS OF ANNELID MORPHOLOGY. 75 



show that if the foregoing discussion of concrescence 

 has any weight the trochophore cannot possibly be 

 an ancestral larval form, but is one that has undergone 

 very great secondary modification. For concrescence 

 takes place throughout the trunk-region ; and if the Hne 

 of concrescence represent the original line of closure of 

 the protostome the trunk cannot be of later origin than 

 the head, since, by the hypothesis, the ancestral radiate 

 body gave rise, by transverse elongation, to both head 

 and trunk. From which it follows that the suppression 

 of the trunk-region, which is the essential feature of the 

 trochophore, must be a secondary matter. In other 

 words, the anterior part for some reason develops more 

 rapidly than the posterior part, which lags behind and 

 only makes its appearance after the anterior part has 

 acquired highly developed organs of locomotion, sensa- 

 tion, and coordination. Strong confirmatory evidence 

 of this view appears to me to be afforded by the follow- 

 ing facts : If the suppression of the trunk region be a 

 secondary character, we should expect to find in the 

 larva some rudiment of the trunk, present, but in an 

 undeveloped state ; and, in point of fact, I believe such 

 a rudiment is always present. If we examine the poste- 

 rior portion of the trochophore we find on each side of 

 the body, near the end of the alimentary canal, a small 

 group of cells — or, it may be, a single cell ('* primary 

 mesoblast "), lying in the cavity of the body. It is this 

 cell or group of cells that in later stages gives rise to 

 the mesoblastic band — i.e. to the basis of one-half of 

 the trunk. The trunk is not present, but its germ is ; 

 and hence it is not strictly correct to say that the tro- 

 chophore represents the head alone. It is a highly de- 



