420 Dr. H. Ludwig on the 



day, we notice that the right foot projects from the surface of 

 the body a little in advance of the left, which again is trace- 

 able to the fact that the plane of symmetry of the Holo- 

 thmian assumes the oblique position with regard to the plane 

 of symmetry of the larva which has already been mentioned. 

 The nivscuJahire of the young feet arises in immediate pro- 

 longation of the musculature of the radial vessel, exclusively 

 in the shape of longitudinal muscle-fibres, on the outer sur- 

 face of the pedal vessel, and originates, precisely like the 

 muscles of the radial vessels and the tentacles, from the cells 

 of the epithelium of the hydrocoele. As early as the tenth 

 day (therefore even before the appearance of the muscle-fibres 

 in the corresponding radial vessel) the longitudinal muscle- 

 fibres form a fine unilamellar sheath, which is still absent in 

 that section of the pedal canal only, which very much later 

 bulges out to form the pedal ampulla. At the point of origin 

 of the pedal vessel from the median ventral radial vessel a 

 valvular arrangement is indeed present, but much more feebly 

 developed than the similar valves of the tentacular canals. 



A third foot does not make its appearance until the forty- 

 fifth day. It arises in front of the two primary feet, always 

 lies to the left of the median plane, and, like the others, 

 receives its water-canal from the median ventral radial vessel, 

 which consequently now supplies two left feet and a right 

 one. In the meantime, from the proximal portion of the 

 first two pedal canals, there have arisen ampulliform expan- 

 sions into the body-cavity. 



On the eighty-fourth day a. fourth foot has come into exist- 

 ence, which likewise derives its water-canal from the median 

 ventral radial vessel. It lies still further towards the front 

 than the third, nevertheless not to the left but to the right. 



A i'urther increase in the number of feet does not take place 

 until the hundred and eleventh day. Thcffthfoot, however, 

 w hich then appears, no longer belongs, like its forerunners, to 

 the median ventral radial vessel, nor even to the ventral 

 surface at all, but arises on the left ( = ventral) side of the 

 left dorsal radial vessel, and, moreover, in the region of tiie 

 anterior half of the body. Ilie same two radial vessels, 

 therefore, are now taking part in the formation of feet, which 

 also in the formation of tentacles in so far preceded the rest of 

 the radial vessels that they were the first to furnish their 

 definite number of two tentacles each. 



The Polian vesicle lies, contrary to the position attributed 

 to it by Selenka in his figure, not in the right half of the 

 body, but without exception in the left, and, indeed, invariably 

 in the left dorsal interradius, and consequently in the abso- 



