276 On the Emlryology &c. 0/ Yoldia llmatula, Say. 



mantle-chamber. Anteriorly the gills gradually diminish in 

 size and finally disappear. Posteriorly they are attached to 

 the wall that separates the two siphons. The mantle-chamber 

 is thus divi-ded by a movable partition into a ventral chamber, 

 opening through the inhalent siphon, and a dorsal chamber, 

 opening through the exhalent siphon. 



In young specimens, in which the brown gills are visible 

 through the shell, the movements of the gills may be observed. 

 They are gradually pressed ventrally, probably by the blood 

 forced into them, the water passing between the gill-plates as 

 they descend. This is followed by a quick contraction of 

 the suspensory membranes (fig. 6, gs)^ accompanied by a 

 vigorous discharge of water through the exhalent siphon as 

 the dorsal chamber diminishes, and a corresponding influx of 

 water through the inhalent siphon as the ventral chamber 

 enlarges. 1'he movements of the siphons accompanying the 

 movements of the gills are very conspicuous, and have been 

 mentioned by Brooks (1). The movements are more or less 

 rhythmic, the time varying with the needs of the animal. 



The currents of water are probably primarily for respiratory 

 purposes, but they aid in clearing the mantle-chamber from 

 the dirt that is constantly finding its way in, and more espe- 

 cially from the faeces, which, being so largely composed of 

 sand and mud, would otherwise drop into and soon clog the 

 mantle-chamber. Besides creating currents of water, these 

 movements may aid in causing the exchange of blood. 



The inner, pericardial, and outer, mantle-chamber, ends of 

 each excretory organ lie very near each other. Pelsner (6) 

 has described the genital duct as opening into the excretory 

 organ near its pericardial opening. In all the cases that I 

 have examined the genital duct bends ventrally, when 

 almost in contact with the inner end of the excretory organ, 

 meets the end and opens with it. The common opening of 

 the excretory organ and genital duct into the mantle-chamber 

 is elongated antero-posteriorly, and seems to represent a fusion 

 of the two rather than an opening of one duct into the other. 



No distinct separation into cerebral and pleural ganglia 

 has been observed. 



The otocystic canals have not been traced to the exterior, 

 but they are distinctly visible near the otocysts. Their 

 meaning is not clear to me, as the otocysts seem to be entirely 

 closed off at an early age. 



In closing, attention will be called to two sensitive areas on 

 each mantle-margin. These lie opposite the extremities of 

 shell-stripes that run from the beaks to the ventral margin, 

 one anterior, the other posterior. The anterior area is in the 



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