REPORT ON THE MOLLUSCA. 9 



latter, indeed, is formed by the edge of the mantle itself, while the open inhalent siphon 

 in MaUetia is formed by the internal fold of the mantle, just like the exhalent siphon 

 to which it is attached. 



At the base of the inhalent siphon, where the retractor muscle of the siphons 

 is inserted, there exists, ventrally, and at the left side only, a long extensile tentacle [j). 



In MaUetia obtusa, Sars ( Yoldia), G. 0. Sars has already noted the presence 

 of pallial tentacles ; ' but, according to his description, each lobe of the mantle bears 

 one (" tentaculis singulis "). 



Brooks was the first to point out that this appendage was not paired in Yoldia"' 

 but he speaks of it as existing on the right side. I have examined Yoldia isonota 

 on this point, and have ascertained that this tentacle is situated sometimes on the left 

 lobe of the mantle, sometimes on the right, but that it is never paired. This may 

 possibly be the case in MaUetia, but I cannot say so positively, as I had only one 

 specimen for examination. 



Does there exist among other Pelecypods any organ comparable to this tentacle in 

 MaUetia and Yoldia ? I have nowhere discovered any notice of similiar structure. 

 I may therefore call attention to an organ which I observed in Macoma balthica, 

 L. sp. (TeUina), but not in all the species bearing the name of Tellina, during my first stay 

 at the Marine Zoological Laboratory in Winiereux (1884)," and which I have seen again 

 since. As this arrangement has never been described, to my knowledge, I represent it 

 here (PI. I. fig. 9, j). 



It is an organ in the form of a tuft, composed of a considerable number of small, 

 short, cylindrical tentacles radiating from the same point ; it is situated at the base of 

 the inhalent siphon, on the retractor muscle of the siphons ; and is always present 

 on each half of the mantle, the left as well as the right. 



The situation of these tufts in Tellina balthica resembles that of the tentacle 

 in Yoldia and MaUetia. The innervation is ^Iso similar ; the nerve springing from the 

 visceral ganglion, and taking its course to the margin of the mantle, gives rise to a cord 

 which terminates in a ganglionic swelling at the base of the tuft in Tellina, or exhibits 

 this swelling on its course at the base of the tentacle, in Yoldia * and MaUetia. 



As to the use of these tufts. Brooks attributes a sensory function to the tentacle in 

 Yoldia. There can, indeed, be no doubt about this point, either with regard to Yoldia 

 or Tellina and MaUetia, when we find a ganglionic swelling, in which a nerve terminates, 

 at the base of the organs in question. 



1 On some Remarkable Forms of Animal Life, i. (1872) p. 25, pi. iii. fig. 20, c. 



^ On an organ of special sense in Lamellibranchiate genus Yoldia, Proc. Am. Ass. Adv. .^ci. for 1874, t. xxiii. 

 (1875) part i. p. 81. 



3 It gives me pleasure to thank Professor A. Giard, director of this laboratory, for the ccrdial and hospitable 

 manner in which he has repeatedly welcomed me there. 



* Brooks, loc. cit., p. 82. 

 (ZOOL. CHALL. EXP. — PART LXXIV. — 1888.) l^CCe 2 



