58 



form enlargement is formed. The external nerve gives 

 off an abundant series of branches which pass outwards 

 to the extreme mantle edge; between external and median, 

 and to a less extent between median and internal nerves, 

 there are numerous anastomosing branches. These anas- 

 tomoses are particularly abundant in the neighbourhood 

 of the branching of the common pallial trunks. 



Sense Organs. 



The sensory structures in Cardium are : — (1) Sensory 

 epithelial cells (Flemming's cells) in the integument ; 

 (2) visual organs borne by the siphonal tentacles ; (3) a 

 pair of otocysts in the proximal limb of the viscero-pedal 

 mass. 



(1) The sensory epithelial cells are found over the general 

 body surface, but are more abundant on the mantle edge 

 and on the siphons. On the latter they are present in 

 groups in the depressions at the free extremities of the 

 tentacles borne by the outer wall.* They are either spindle- 

 shaped cells with the nucleus at the middle and a bundle 

 of long hairs on the free extremity, or elongated cells with 

 the nucleus at the base and the free end swollen out into 

 a disk, which bears short hairs. Probably they act both 

 as tactile and as olfactory organs, the latter function being 

 subserved more especially by the cells on the mantle edge 

 and siphons. 



(2) The structures generally regarded as visual organs 

 (fig. 36, PI. VI.) are found on the summits of the tentacles 

 borne by the outer walls of both siphons. At the free end 

 of each tentacle there is a small depression. The lip of 

 this depression which faces the siphon is gently rounded, 



*Flemming. Untersuch. u.d. Sinnesepithelien der Mollusken. Arcliiv 

 f. Mikr. Anat. Bd. VI., pp. 439—471, 1870. 



