274 Mr. G. A. Drew on the 



greater part of the external surface of tlie embrvo. An the 

 body of the larva elongates posteriorly these cells are crowded 

 forward to form the velum, which, however, does not seem 

 to be cast off. 



Through these forms we may, perhaps, trace an homology 

 between the test of Yoldia and tlie velum of other forms. In 

 tliis connexion it is interesting to notice that in a few forms 

 the velum is known to be cast away. This was observed by 

 Sigerfoos (8) for Teredo and by Wilson (9) for Polygordius. 



The condition presented by the mouth and anus both 

 opening through the blastopore is interesting. It may be 

 that the blastopore offers the only available place for the anus 

 to open, or it may stand in relation to forms on the one hand 

 that have the blastopore persisting as the mouth, and on the 

 other hand to forms that have the position of tlie blastopore 

 occupied by the anus. 



The formation of the cerebral ganglia from the walls of 

 invaginations deserves special mention, inasmuch as it seems 

 to be the first case reported for the Lamellibranchiata. 



Anatomy and Habits. 



Yoldia Umatida lives in soft roud or ooze, in wiiich it 

 moves about by means of its muscular foot, wiiich is so 

 modified that ils edges can be turned outward and so form a 

 first-rate anchor. So efficient is the foot in burrowing that a 

 specimen placed upon the surface of the soft mud in which it 

 lives will completely bury itself with two thrusts of the foot. 

 The foot is very sensitive and is moved with wonderful 

 rapidity. In every way it shows itself to be nicely adapted 

 for burrowing. It seems very hard to imagine that it could 

 possibly be used as a creeping-organ. 



From the posterior part of each external palp there arises 

 an elongated appendage, known as the palp -appendage, 

 ^vhich can be protruded to a distance considerably exceeding 

 the length of the shell. This appendage is folded longitu- 

 dinally to form a groove that runs from its tip to its point of 

 attachment. 



When the animal is feeding the shell is slightly tipped 

 ventrally from the perpendicular, and about two-thirds of its 

 anterior end is buried in the mud. The palp appendages are 

 thrust out of the shell, and one at least bends over and inserts 

 its tip into the mud. The cilia lining its longitudinal groove 

 inmicdiately begin to elevate the mud, which is rich in living 

 organit^ms. The stream of particles passing along the groove 

 is laige enough to be distinguished at a distance of some feet. 



