156 COMPARATIVE ANATOMY. 



Sabellidae (Branchiomma) ; in them the eyes are either placed 

 in large numbers on the pinnate branches of the branchial filaments, 

 or at their ends only. In other Annelides there is a similar change 

 in position as compared with the primitive one. In many there are 

 eyes at the posterior end of the body, as well as on the cephalic 

 segmeiit ; and finally, in the genus Polyophthalmus there is a pair of 

 eyes on each metamere, in addition to those on the head. We here 

 find an arrangement which is not only of importance as bearing on 

 the estimation of the metameres, but is also a proof that visual 

 organs may be developed at points which in other forms only carry 

 sensory organs of a lower kind. 



Auditory Organs. 

 § 126. 



We consider as auditory organs in the Vermes organs which, as 

 in the Ccelenterata, consist of a vesicular capsule, in which there is a 

 firm, large concretion, or a number of smaller ones. The wall of the 

 capsule is frequently invested with cilia, as may be seen from the 

 trembling movements of the auditory stones (otoliths). The difficulty 

 of making out the nerve-branches in the lower Vermes — in which, 

 indeed, these organs are most largely distributed — has generally 

 caused the connection of the auditory vesicles with the nervous 

 system to be missed. 



These auditory vesicles are generally unpaired in the Turbellaria, 

 in species of Monocelis, Convoluta, Proporus, Derostomum. They 

 generally lie close to the cerebral ganglia, and are found as a rule in 

 those genera which are devoid of eyes or eye-spots. In the Nenier- 

 tina they have only been observed in some cases (CErstedia). In 

 the rest of the Platyhelminthes these auditory vesicles are not, 

 apparently, present, and they are also wanting in the Nematodes. 



Only in the Annelida do they appear again, where they are 

 paired, and as a rule placed at the sides of the brain (Alciopidie 

 Arenicola, Fabricia, Amphiglena, etc.). 



Alimentary Canal. 

 § 127. 



The alimentary canal of the Vermes forms a tube, which is 

 either embedded in the parenchyma of the body, or, when a ccclom 

 is present, in it^ it has a general adaptation to the form of the 

 body. The mouth lies, as a rule, at the anterior end of the body, and 

 is always placed on the ventral surface. Where an anus is present, 



