GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



325 



hesitate to attribute to them the significance of transit ionary forms. 

 We have already pointed out that the spines which cover the body 

 of the Amphineura (Figs. 147 B, and 6-8, p. 10) show striking agree- 

 ment in their origin with the setae of the Chaetopoda and it was 

 mentioned that some authors had regarded this as proving relation- 

 ship between the Amphineura and the Annelida. We do not attach 

 any great importance to this resemblance, since these spines are 

 found irregularly distributed over the body, while the setae of the 

 Annelida are, as is well known, very regularly and segmentally 

 arranged. Spines also occur in some forms while the related forms 



Pig. 147. — A, Proneomenia aglaoplieniae, wound round a branch of Aglaophenia >■ v, 



indicates the anterior end of the body. B. anterior end of the same animal seen 

 from the ventral side, and showing the oral aperture and, behind it, the aperture of 

 the pedal gland and the ventral groove (after Kowalevsky and Marion). 



show no such structures. As an example of this we would recall the 

 Turbellarian described by v. Graff, Enantia spinifera. Here we 

 have true euticular spines which can only be considered as analogous 

 to those of other animals, but in connection with the origin of such 

 structures this illustration is of interest. 



In comparing the spines of the Amphineura with the setae of the 

 Annelida it must be remembered that we can certainly not derive 

 the Mollusca from such highly developed forms as the Chaetopoda. 

 The most primitive Annelida, however (the Archi-Annelida), have no 

 setae, but even these are certainly too much differentiated to serve as 



