Mr. P. H, Gosse on Cardium exigiumi. 257 



XXIV. — Cardium exiguum : — its Siphons and its Bi/ssus. 

 By riiiLip II. Gosse, F.R.S. 



[With a Plate.] 



A MINUTE Cockle {Cardium exiguum), about one-fourth of an 

 inch in diameter, and of a pure white hue, was dredged by me in 

 Weymouth Bay, May 13th, 1855, and was deposited in one of 

 my aquaria. For some time after I had domiciled it I saw no 

 more of it, and supposed it was lost ; but one day my attention 

 was arrested while looking with my lens through the glass side, 

 along the edge of the bottom-rubbish, by an object which I 

 knew not what to make of. From the midst of the floccose 

 matter a very minute bladder was projected, the motions of 

 which were so vivacious as to cause me no little surprise and spe- 

 culation as to what manner of thing it might be. After vainly 

 trying to decipher it by mere gazing, I ventured carefully to 

 clear away some of the rubbish on each side with a pin-point 

 fastened to a stick ; when I discovered my tiny friend, the Cockle. 

 No trace, it is true, was now to be seen of the bladder, but after 

 a few minutes I saw it again, and understood the mystery ; not 

 indeed all at once, but by degrees, and by repeated examinations. 

 The facts I thus learned I will now record. 



In the great spinous Cockles (C aculeatum, &c.) the ejecting 

 or anal siphon is formed closely like the receiving or oral one, — 

 a simple orifice, surrounded by filiform tentacles. But in this 

 pigmy species the anal orifice is crowned by a semi-elliptical sac, 

 which at the instant of opening the valves for the renewing of 

 respiration is projected with a jerk. This sac is composed of 

 membrane of the most extreme delicacy, and of such transpa- 

 rency that it would be utterly invisible but for rows of minute 

 opake- white dots that run down it longitudinally. It terminates 

 in a circular aperture, whose width is about half that of the 

 greatest diameter of the sac ; but from the sensitiveness and 

 contractility of the membrane, the form and dimensions of the 

 orifice slightly vary. Its edges are not in the least thickened, 

 and they are with the greatest difficulty detected, except by the 

 termination of the macular lines just mentioned. In some cir- 

 cumstances it is protruded to a much greater extent than in 

 others, forming a very elongated ellipse, and extending to the 

 length of ^th of an inch, or little less than the transverse dia- 

 meter of the valves. I for a time thought it was projected by 

 the evolution of its walls ; but on more careful examination, I 

 saw that the sides collapsed into a wrinkled thread when the 

 jet ceased, and were instantly distended, with force, when it was 

 renewed. 



The movements of this organ, though not extensive, are 

 Ann, ^ Mag. N. Hist. Ser. 2. Vol. xviii. 17 



