SEA-FISHERIES LABORATORY. 937 
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 Carduum, and in C. muticwm, 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,+ 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, Pl. 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 lne 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, Reeve. Journal Roy. Coll. of 
Science, Imp. University, Tokyo. Vol. VI., pt. 4, pp. 279—285, Pl. IX., 1894. 
+ Uber die sogenannten Augen von Tridacna, &c. Zeitsch. f. wissensch. 
Zool. Bd. LXVI., pp. 270—88, Pl. XXI. (English translation in Ann. Mag. 
Nat. Hist., 1888, pp. 485—52), 
