Mabch 25, 1910] 



SCIENCE 



471 



The Morphology of the Stmm-hladder in Teleosts: 

 Henbt C. Tbact, Brown University. (Intro- 

 duced by A. D. Mead.) (Presented by title.) 

 The most important types of swim-bladders are, 

 first, primitive swim-bladders with an open pneu- 

 matic duct and undifferentiated epithelial lining, 

 and, second, the highly specialized type without 

 ■duct but with the so-called " oval " on its dorsal 

 wall. 



Two exceptional types are probably to be con- 

 sidered transitional forms. One is the swim- 

 bladder of the eel; the duct is enlarged into a 

 ■capacious chamber, but its oesophageal connection 

 is much reduced. The duct is lined with flat 

 «pitheliiun under which is a rete mirabile. 



The other transitional type is found in toadfish 

 {Opsanus) and a few other forms. It has lost its 

 ifiesophageal connection, but is divided into an an- 

 terior and a posterior chamber by a transverse 

 partition, through which is a round opening. The 

 structure of the walls of the posterior chamber is 

 like that of the duct of the eel. This chamber 

 develops directly from the embryonic pneumatic 

 •duct. 



From the posterior chamber of the swim-bladder 

 ■of the toadfish the transition to the oval may be 

 ■considered to have taken place by an approxima- 

 tion of the partition to the posterior wall of the 

 ■organ. The red gland develops by a progressive 

 .differentiation of the epithelial lining. 



iCiliation of the Palps of the Acephala: J. L. 



Kellogg, Williams College. 



The known function of the ciliated inner sur- 

 faces of the palps of bivalves is to transport food 

 ■particles from gills to mouth; but they have been 

 found also to possess the power of directing unde- 

 •sirable materials, such as mud, on to ciliated 

 tracts that carry them out of the body. 



The inner palp surfaces are found to possess 

 ■four distinct currents: one across the folds to the 

 mouth; a second in the opposite direction, on the 

 ^ventral palp margin; a third set of tracts on the 

 jfaces of the palp folds, from their ventral to their 

 dorsal ends; and a fourth set, deep in the grooves 

 ■between folds, from dorsal to ventral ends. 



The function peculiar to each of these was fully 

 'determined during the past summer in several of 

 the large forms of Puget Sound. That of the 

 fourth set is especially interesting. These tracts 

 are entirely covered when the animal is feeding. 

 "They are exposed by a peculiar movement of the 

 iolds when a large quantity of material is brought 

 Ao the palps, as is the ease in muddy water, the 



entire mass being led to tracts that convey it 

 from the body. The fate of particles brought to 

 the palps is determined not by their nature — 

 whether suitable for food or not — but solely by 

 their volume. 

 Parallel Development in Tropical Trematodes: 



H. S. Pbatt, Haverford Ckillege. 



The digenetie trematodes as well as other in- 

 ternal parasites have probably in their phyletie 

 history followed somewhat different rules of 

 descent from those of other animals. The fact 

 that they live inside of other animals and have 

 also a very complex life history must affect their 

 phyletie development most profoundly, and in two 

 ways : ( 1 ) The possibilities of migration are very 

 much limited. (2) The environment of the para- 

 sites being extremely uniform and subject to rela- 

 tively little variation there is a corresponding 

 uniformity of structure in the parasites them- 

 selves. Thus we see that although there are sev- 

 eral thousand species of digenetie trematodes in 

 existence living in all parts of the world they are 

 astonishingly alike in structure — so much so that 

 until quite recently all of the thousand or more 

 species of distomes were included in the single 

 genus Distomum. The monogenetic trematodes, on 

 the other hand, which are external parasites and 

 have consequently a very much simpler life history 

 and a much more varied environment show a much 

 greater variety of structure, although they count 

 fewer species. 



These facts make it probable that where there 

 are apparently related species of digenetie trema- 

 todes living in widely separated localities the fact 

 that they possess the same or similar structural 

 features does not necessarily indicate that there 

 is a close genetic relationship between them. 

 They have not necessarily inherited their pe- 

 culiarities from a common ancestor even when 

 they are so much alike that they are classified in 

 the same genus. But they are undoubtedly in 

 very many cases descended from different an- 

 cestors and have reached their present structural 

 condition by traveling along parallel or converging 

 lines of descent. 



These facts are well illustrated by the several 

 species of digenetie trematodes belonging or allied 

 to the genus Eelicometra which were found in 

 certain fishes in the Gulf of Mexico at Tortugas, 

 Florida, and also occur in the Mediterranean Sea. 

 That the species of this peculiar genus are thus 

 taken as an indication, not that they necessarily 

 bear a close genetic relationship to one another. 



