Eyes of MoUuscs and Arthropods. 60 9 



These suppositions, which have been merely outlined, so rim into 

 eacb other that they allow the interpohitiou of subdivisions, and a much 

 more extended treatmeut; this will, however, suffice for my purpose, 

 for it euables me to arrive at au important eonchisiou of practical value : 

 if, for certain reasons, the same combiuation of light vibrations, aeting- 

 lipon the same visual elemeuts, produces differeut perceptions, or 

 dififerent iuteusity of reflex actious, theu it becomes actually impossible 

 to determiue the relative fuuctional powers of two eyes, by the differ- 

 ence in eflfect produced by the same image, or by the same combiuation 

 of light vibrations : and, eonversely, ueither can one determine what 

 efifect will be produced by the perception, throug-h equivalent eyes, of 

 the same image. There are pleuty of cases to illustrate both sides of 

 this proposition ; therefore when a person attempts to judge of the per- 

 fection of au eye by experimeuts aloue, bis deductions are liable to be 

 wrong, or at least to contain an abuudance of uncertain Clements. 



The necessity of keeping these facts in mind is only too evident in 

 the present instance. By experiment alone, we woiild be led to suppose 

 that Ackula had eyes at least as complicated as those of Area ; but 

 we are surprised to find that there are only a few scattered ommatidia, 

 which would entirely escape the notice of one who had not seen them 

 better developed elsewhere. But if we are led into such false concep- 

 tions by experimenting upon organs, which, the first glance teils us, are 

 widely dififerent in structure, how can we determine, by experiment 

 alone, the relative perfection of organs which are nearly alike in histo- 

 logical structure? 



Cardiiim. 



The rather massive sipho of C. edule is beset with a double row 

 of tentaci es, which, in the expanded condition, form a radiating fringe 

 around each of the siphonal openings. On the anterior and posterior 

 side, the fringe attains its greatest development , while , between the 

 anal and oral opening8,it is reducedto a few, small, scattering tentacles ; 

 those on the anterior side of the oral opening are the largest of all, 

 and one of these, from which the remaining ones dififer but slightly, we 

 shall select for description. It is extended directly away from the oral 

 opening , with the exception of the expanded tip which is bent nearly 

 at right angles towards the sipho (PI. 31, fig. 113 . On the side 

 a w a y f r m t h e 1 a 1 1 e r , is a semi-cir cular band of bro wu pigment cells ; 

 the remainder of the tentacular hypodermis is colorless. 



MittheiluEgeii a. d. Zoolog. Station zu Neapel. Bd. VI. 4| 



