VII MOLLUSCA^THE SENSORY ORGANS 167 



fibres of the other aesthetes of the shell-plate, between the tegmentum 

 and articulamentum to the surrounding pallial tissue, or else pene- 

 trates the articulamentum. 



The significance of the separate constituent parts of the testhetes and their 

 fibrous strands is not yet certainly known. It is probable that they are innervated 

 from the dorsal lateral branches of the pleuro-visceral cords. It is even not known 

 whether the fibrous strands are their nerves, or whether the clear fibres running 

 through them are long sensory cells whose nuclei may lie between the glandular 

 cells, and in connection with nerve fibres. 



We are perhaps justified in assuming that the esthetes are merely modifications 



hf "^ ^' 



Fig. 143.— Section of the tegmentum of Chiton laevis showing an aesthete (after Blumrich). 

 ink, MicrKstliete ; per, peridstnicuiii ; sk, principal a'stlii'te ; t, tegmentum ; ds, cells resembling 

 glandular cells ; ///, clear fibres ; /s, tibnius straii<l ; c, chitiiious cap. 



of the spines with their papillae and formative cells, which are so common in tlie 

 integument of the Chitonidoe. The chitinous cap would then represent part of the 

 chitinogenous base of the spine. 



The sensory nature of the aesthetes is rendered highly probable 

 by the circumstance that in a few species of Chiton individual megal- 

 sesthetes are transformed into eyes. 



Each eye is furnished with a pigmented envelope, which is pene- 

 trated by the micrsesthetes, and outwardly covered by an arched 

 layer of tlie tegmentum which forms the cornea. Under this is a 

 lens, and under this again a cell layer, Avhich is regarded as a retina, 

 and to which is attached a fibrous strand (optic nerve 1) corresponding 

 with the fibrous strands of the ordinary aesthetes. 



B. Auditory Organs. 



All Mollusca except the Amphiiieura possess auditory organs, Avhich 

 appear very rarely in the embryo. They take the form of two almost 



