Fkbeuaby 2, 1900.] 



SCIENCE. 



173 



visceral mass may be transferred to the 

 inner surface of the mantle, referred to 

 above. If not, it is discharged through 

 the branchial siphon on the contraction of 

 the adductor muscles. 



(3) Gills, as is well known, collect mat- 

 ter and conduct it forward along a ventral 

 groove to the palps. 



(4) The inner surfaces of the palps bear 

 ciliated ridges. In a peculiar manner, the 

 cilia of the ridges drive particles to the 

 mouth. Around the entire margin of the 

 palp is a tract on which the ridges do 

 not extend. Its cilia swiftly carry matter 

 away from the mouth, throwing it off from 

 the posterior, free tips of the palp, into the 

 branchial chamber. Eventually this may 

 be discharged from the body as previously 

 described. By muscular movement these 

 edges may be applied to any surface within 

 reach, sweeping it clean of particles which 

 it may be bearing, and casting the material 

 into the branchial chamber. 



In Yoldia, which lives in soft mud, the 

 mechanism of the palp, though it performs 

 the same general function, is very much 

 more complex. The plate gills, also, which 

 collect particles with amazing swiftness, 

 may, if desirable, transfer them to the 

 palps, or may be rid of the material by al- 

 lowing it to pass up through a peculia'rly 

 constructed passageway between two plates 

 to the epibranchial chamber, whence it is 

 carried from the body by the stream of 

 water continually being discharged through 

 the epibranchial chamber. 



This mechanism is very different in differ- 

 ent forms, due to structural peculiarities of 

 regions bearing cilia, and to the habits and 

 needs of various species. We have facts 

 sufficient to show that, without hindrance 

 to the respiratory processes, Pelecypods have 

 the power of collecting food by means of 

 a very complex mechanism ; or at will, 

 through the muscular movement of certain 

 ciliated surfaces, of removing any material 



which it may be desirable to prevent from 

 entering the digestive tract. Data is being 

 collected for a description of this mechanism 

 in as many species as possible. 



Observations upon the development of Phasco- 



losoma. J. H. Geeottld. 



The forms that were used for investiga- 

 tion were Phascolosma gouldii, Ph. vulgare 

 (Blainv.) and Ph. elongatum (Keferst.) The 

 ova which are ready for maturation are 

 swept into the nephridia by the action of 

 the cilia of nephrostome and of the internal 

 surface of the nephridium. The eggs that 

 are found within the nephridia, prior to 

 their ejection through the nephridiopore, 

 have the spindle of the first polar globule 

 with ten rod-shaped chromosomes already 

 formed. The astrosphere of the male pro- 

 nucleus precedes the nucleus in the migra- 

 tion of both toward the center of the cyto- 

 plasm ; it usually divides before the union 

 of the two pronuclei. The segmentation 

 nucleus contains twenty chromatic fila- 

 ments which split longitudinally. 



The alternating directions of the cleav- 

 age planes as far as forty-eight cells are 

 identical with those in corresponding stages 

 in the eggs of annelids, most molluscs, etc. 

 The most striking peculiarity in Phasco- 

 losoma is the large size of the first set of 

 ' micromeres, ' which in quadrants A , B and 

 Care distinctly larger than their sister cells 

 at the vegetative pole. The ' macromeres ' 

 throughout the course of cleavage are of 

 small size. 



The mesoderm is derived from d*, which 

 in the one instance observed divided in 

 harmony with the regular alternation in 

 direction ; its posterior derivative divided 

 immediately to form a second mesoblast. 

 In the divisions of the cells of the somatic 

 plate deviations from the rule of alterna- 

 tion in the direction of cleavage were ob- 

 served. 



The rosette, cross and intermediate cells 



