518 



of these organs are sensitive to mechanical stimulation. The un- 

 paired tentacle seems to be homologous to one of the ordinary mar- 

 ginal tentacles. 



2) The foot is a highly modified burrowing organ. 



3) The palp-appendages are very active collectors of food. 



4) The gills of Y. limatula are used in pumping water, for which 

 function they are well fitted by their structure. The gills of Nucula 

 may function in the same way, to a less extent. 



5) The genital ducts join the outer, and not the inner ends of 

 the excretory organs. 



6) The otocysts of Y. limatula are not connected with the surface 

 by canals. Canals, apparently in process of degeneration, are present 

 in adults. 



Embryology. 



1) The embryos of N. delphinodonta are carried for many days, 

 in mucous-like cases that are attached to the posterior ends of the shells. 



2) The original surface cells seem to give rise to the tests, the 

 cerebral ganglia, the apical plates, and to new ectoderms inside of 

 the tests. 



3) The cerebral ganglia of Y. limatula are formed as invaginations 

 from the surface. 



4) The otocysts of Y. limatula contain otoliths before the tests 

 are cast. 



5) The test, at least part of the apical plate, and the stomodaeum 

 to the position of the definitive month, are all cast away. 



6) It seems that typical molluscan larvse may have been developed 

 from ancestors resembling the embryos of Yoldia and Nucula in form 

 and structure. 



Literature cited. 



1) Flemming, Die Haare tragenden Sinneszellen in der Oberhaut der 

 Mollusken. Arch. f. mikrosk. Anat., Bd. 5, 1869. 



2) Kellogg, A Contribution to our Knowledge of the Morphology of 

 Lamellibranchiate Mollusks. Bull. U. S. Fish. Com., Vol. 10, 1890. 



3) Kowalevsky, Etude sur l'embryogene du Dentale. Ann. du Musee 

 d'Hist. nat. de Marseille, Zool., T. 1, 1883. 



4) Lacaze-Duthiers, Histoire de l'organisation et du developpment du 

 Dentale. Ann. Sc. nat., Ser. 4, T. 7, 1857. 



5) Mitsukuri, On the Structure and Significance of some Aberrant 

 Forms of Lamellibranchiate Gills. Quart. Journ. Micr. Sc, Vol. 21, 

 1881. 



6) Patten, The Embryology of Patella. Arb. Zool. Inst. Univ. Wien, 

 Bd. 6, 1886. 



7) Pelseneer, Contribution ä l'etude des Lamellibranchs. Arch, de 

 Biol., T. 11, 1890. 



