THE TBOCHOPHOEB STAGE OF FRESH-WATEB LAMELLIBBANCHIA. 39 



Quite near the mesoderm-bands, at their anterior end, an organ 

 is developed which makes the comparison of the Laruellibranch 

 Trochophore with the Annelidan larva almost complete, viz., the larval 

 or liead-kidney. According to the observations of Hatschek, who 

 tirst discovered this organ, it is a long tubular structure with a 

 narrow cavity (Fig. 18 n). In later stages it lengthens further; its 

 external end becomes applied to the ectoderm and opens on to the 

 exterior through a hue aperture. Its cavity is lined with fine cilia 

 directed outwards, and its inner end seems to widen out like a funnel 

 towards the body-cavity. This organ, which was also observed by 

 Zieglek in Cyclas, thus possesses all the peculiar characteristics of 

 the head-kidney found in the Annelidan larva. The same primitive 

 excretory organ is also found in the Trochophore of the Gastropoda 

 (p. 136). 



Hatschek thinks it probable that, in Teredo, the canal of the larval kidney 

 (■"liimunicates with the hody-cavity, but Zieglbr was not able to convince 

 himself that this is the case in Cyclas. In the latter, the inner end of the 

 canal is lost in a mass of mesoderm-cells. Ziegler assumes that the canal 

 i> formed of large perforated cells such as occur in this organ in the Gastro- 

 poda.* 



B. The Trochophore stage of Fresh-water Lamellibranchia. 



Among the fresh -water Lamellibranchs, as has already been 

 pointed out (p. .'50), only Dreissensia has a free-swimming larva, 

 which, indeed, exhibits exactly the same characteristics as are found 

 in the Trochophore ami later stages of the marine Lamellibranchia. 

 For this reason, the larva of Dreissensia has already been considered 

 in the previous section (Fig. 17, p. .'55). A special resemblance 

 exists between the larvae of Dreissensia and those of Mytilus as 

 described by Wilson (No. oil). 



The conditions found in Dreissensia form an interesting contrast to those 

 met with in other fresh-water Molluscs and to those of fresh-water Annelida, 

 Turbellaria and Hydrozoa, since all these forms have lost the free-swimming 

 larva. This is explained on the belief that Dreissensia, which is a near rela- 

 tion of Mytilus, has migrated from the sea into fresh-water only at a recent 

 date, and has consequently retained the free-swimming larva together with 

 other characteristics of a marine form, v. Martens (No. 34). 



[The recent observations of Meissenheimer (App. Lit. Gastropoda, No. 

 XVIII.) on the head-kidney of Limax tend to show that there is no com- 

 munication between the lumen of this organ and the body-cavity. On the 

 other hand, Stauffacher (No. VII.) maintains that, in Cyclas, this organ 

 does communicate with the primary schizocoele. — Ed.] 



