498 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Reproductive Organs of Valvata piscinalis,* — Dr. P. Garnault 

 gives a description of the reproductive organs of Valvata piscinalis ; at 

 first sight they aj)pear to differ a good deal from those of other andro- 

 gynous Molluscs, but the resemblances are seen when the relations of 

 the parts are laid down as in a ground-plan. At the same time they are 

 distinguished by the fact that there is a communication between the 

 efferent canal and the copulatory pouch, and this ensures self-fecundation 

 in cases in which copulation is not effected. Further details and 

 illustrative figures are promised. 



5. Lamellibrancliiata. 



Morphology of Teredo. t — M. A. Menegaux has investigated the 

 homologies of the different organs of this aberrant Lamellibranch. 

 Haviug discovered the anttrior adductor-muscle, he is able to say that 

 it is a dimyarian ; this muscle is very small, is covered by a pallial 

 lobule and separated from the posterior adductor by the rectum and a 

 vessel which accompanies it. The " palettes " are moved by three special 

 muscles, the largest of which arises from the siphonal muscles, the other 

 two are lost in the mantle. The single aorta corresponds anteriorly 

 with the anterior and posterior aortfe of other Lamellibranchs, but after it 

 has passed the posterior adductor it no longer corresponds to the posterior 

 aorta. To the right of the rectum it gives off two lateral pallial vessels ; 

 then it passes slightly to the right, follows the right sij^honal nerve and 

 gives off* a branch to each of the siphons. This asymmetry of the circu- 

 latory system is more apparent than real ; it reminds one of what obtains 

 in Pholas, and the difference is due to the fact that the mantle of Teredo 

 being greatly developed in a longitudinal direction the posterior aorta 

 is of considorable length before it bifurcates to go to the siphons. 



Origin of Unionidse.J — Prof. M. Neumayr has no doubt that the 

 great stock of the Mollusca was originally developed in the sea. Of 

 the fresh-water groups now existing, the most widely distributed and 

 important of the Lamellibranchs are the Unionidae, which appear to be 

 descended from the marine Trigonia. This form has the hinge of the 

 peculiar schizodont type, and, though the hinge-structure is exceedingly 

 variable in the Unionidse, we find on close examination of normal forms 

 that they may be referred to the same type. AflSnity is also shown by 

 the structure of the gills, the separation of the two lobes of the mantle, 

 and the absence of siphons. In both groups the nacreous shell exhibits 

 extraordinary development, there is a strong epidermis and a resemblance 

 in the arrangement of the muscular scars. It is particularly remarkable 

 that in many geologically young Uniones of Pliocene and recent times 

 shell-ornaments appear as retrogressive structures, such as occur else- 

 where only in the Trigonise. 



MoUuscoida. 



a. Tunicata. 



Developmental History of Distaplia magnilarva.§ — In the first of 

 his memoirs on the development of this compound Ascidian, Dr. M. v. 

 Davidoff deals with the maturation of the egg. The structure which is 



* Zool. Anzcig., xii. (1889) pp. 266-9. 

 t Coinptes Rendus, cviii. (1889) pp. 537-8. 



X Anzeig. K, Akad. Wiss. Wien, 1889, p. 4 ; Ann. and M-ig. Nat. Hist., iii. 

 (1889) p. 372. § Mittheil. Zool. Stat. Neapel, ix. (1889) pp. 113-78 (2 pis.). 



