JG-L BULLETIN OF THE UNITED STATES FISH COMMISSION. 



plate, oesophagus, and blind saccular portion of the stomach), the brood 

 begins to assume a gray or bluish color. The dimensions of the valves 

 are now O mn, .lC (about T ^j of an inch) ; their form is almost circular, ex- 

 cept the hinge border, which is straight. As already noted by Lacaze- 

 Duthiers, the hinge at this stage is already provided with teeth. One 

 may now note that the whole animal is withdrawn within the shell from 

 time to time. This is effected principally by the help of a dorsal and a 

 ventral muscle, ds and vs, which originate near the hinge border, and 

 are inserted at the base of the velum. These muscles are formed of 

 branched, attenuated, mesodermal cells, the branches of which traverse 

 the body cavity in various directions. Several of these cells, which ex- 

 tend across from the left to the right half of the shell, have been aggre- 

 gated into a group, and form a distinct adductor muscle, sp. Whenever 

 the larva swims it thrusts the head or velar end of the body out of the 

 shell, and partly turns it outwards over the edges of the latter anteriorly. 

 The preoral ciliary crown consists of a double row of long cilia. If the 

 velar area is viewed from above, the cilia will be found implanted upon 

 two nearly approximated rows of almost rectangular cells. From each 

 of these cells two cilia arise, which in stained preparations may be traced 

 for some distance into the cellular protoplasm. I was unable to detect 

 a post oral ciliary band, although the cephalic extremity of the embryo 

 behind the preoral band is clothed with cilia. The velar area now con- 

 sists in great part of a single layer of very much flattened cells, which 

 can scarcely be defined even when aided by the presence of their stained 

 nuclei; only in the center is there a marked thickening, which projects 

 inwards, composed of distinct layers of ectodermal cells (tp). This is 

 the structure which we have heretofore been designating by the name 

 of velar area [topplaat; German, Scheitclplatte), and from which the 

 supracesophageal ganglion is developed. A longitudinal groove appears 

 to divide this area superficially into two halves, but in consequence of a 

 black pigment which is usually developed in this region, I could not 

 make it out distinctly. Peripheral nerves, which pass outwards from 

 the central velar thickening, such as were observed in the larvae of 

 Teredo by Hatschek, were not encountered by me. The above-men- 

 tioned ectodermal thickening appears to have been noticed in the larval 

 oyster by Davaine, as well as by Lacaze-Uuthiers, but was at first re- 

 garded by both as the oral opening — an error which was afterwards rec- 

 tified by the last-named investigator. 



Together with the other parts of the body the intestinal canal has 

 also progressed in development, the oesophagus, which has been pig- 

 mented with a brown color, has grown longer, and its anterior portion 

 has been widened, and become funnel-shaped. The cavity of the 

 stomach has grown in length, and a constriction divides it into an up- 

 per and a lower portion. From the upper portion on the left and right 

 sides a large round blind sac (/) has been developed, which constitutes 

 the rudiment of the liver, while at the level of the constriction the in- 



