60 



on the pigment patch, though the existence of such is 

 difficult to demonstrate. 



There can be little doubt that the structure so described 

 functions as a very simple eye, capable only of distinguish- 

 ing differences in the intensity of the incident light. Thus, 

 if a shadow be rapidly thrown on a cockle lying in a 

 shallow dish, with the siphons fully extended, retraction of 

 the latter generally follows, and it seems reasonable to 

 locate the sensitive parts in the structures on the tips of 

 the tentacles. Very similar organs are found in other 

 species of Cardium, and in C. muticum, Kishinouye* has 

 described organs on the tip of the siphonal tentacles which 

 have all the characters of an eye — a multicellular lens 

 composed of flattened cells, beneath which is a retinal 

 layer of elongated cells, and beneath this again a pigment 

 layer. The only other conjecture as to the nature of the 

 structures described in the common cockle has been made 

 by Brock, t who supposes them to be luminous organs; 

 but it does not appear that there are any observations in 

 support of this suggestion. 



(3) A pair of otocysts (fig. 34, PI. VI.) are present in the 

 proximal limb of the viscero-pedal mass. They are very 

 small — 0"07 mm. in longest diameter, and can only be ob- 

 served in sections taken in the neighbourhood of the pedal 

 ganglion. They are situated a little way above the latter, 

 right and left of the middle line and internal to the 

 cerebro-pedal connectives. They are probably innervated 

 from the latter. They are oval in form, the longest dia- 

 meters lying parallel to the transverse axis of the foot. A 



*Note on the Eyes of Cardium muticum, Meeve. Journal Roy. Coll. of 

 Science, Imp. University, Tokyo. Vol. VI., pt. 4, pp. 279—285, PI. IX., 1894. 



t Uber die sogenannten Augen von Tridacna, &c. Zeitsch. f. Avisseusch. 

 Zool. Bd. LXVI., pp. 270—88, PI. XXI. (English translation in Ann. Mag. 

 Nat. Hist., 1888, pp. 435—52). 



