352 EDWARD S. MORSE ON 



into each gill-bearing lacune in turn and coursing through each gill lappet. The arrows 

 in 53: 1, will explain the course of this current. Nearly on a, line with the anterior 

 occlusors, are a few short lacunae which have no gill ampullae. In alcohol, as we have 

 seen, the palhum contracts and the region of the gill ampullae is thrown into a strongly 

 bulging fold from which the gill lappets hang and these do not appear to contract (47: 3) . 

 Another sinus turns directly backward from the opening of the main pallial sinus and 

 follows along close to the lateral margin of the pallium sending a series of branching 

 lacunae to the shallow posterior portions of the pallium. This I have indicated as 

 the lateral pallial sinus (53: 1, 1. p. s.) . Beside the main pallial sinus and the lateral pal- 

 lial sinus, there is another which turns abruptly back from the main sinus opening, runs 

 along on each side of the gourd-shaped outline of the perivisceral cavity, inchning 

 slightly toward the median hue, and disappears in a distinct round opening near the 

 posterior lateral border of this outline (53: 1, c. s.). This I have designated the coelomic 

 sinus. It was somewhat difficult to make out the precise limits of the sinus against the 

 dark color of the viscera within. There is seen on the dorsal side a very marked sinus 

 running in the median line from the posterior occlusor muscle to the posterior border of 

 the perivisceral outline. This sinus becomes filled with coloring matter when injected 

 from the peduncular cavity. I have termed this the median sinus (53 : 1, m. si.) . Is this 

 a trace of the dorsal vessel in annelids ? The sinuses and all the lacunae, to the smallest 

 twig, have a central ridge of ciliated epithehum which springs from that portion of the 

 pallium next to the shell. It does not separate the sinuses and lacunae into two distinct 

 channels or tubes. In life, the lacunae, distended by the flow of blood within, appear 

 almost tubular, as seen in L. lepidula (53 : 11, 12) . There is nothing to prevent the 

 currents, which continually flow in opposite directions, from intermixing, yet, in life, this 

 crossing of the currents has not been observed. In 53 : 4, 5, 6, 7, are sketches of transverse 

 sections made through the paUium, intercepting some of the smaller branching lacunae. 

 These sections show the thickness of the pallium next to the palhal chamber, and the 

 extreme tenuity of the membrane separating the cavities of the lacunae from the shell. 

 That the ciliary ridges, though they separate the currents flowing in the sinus, do not 

 separate the sinus into two tubes, may be seen in T. corecouca, where at certain times 

 the sinuses are packed to repletion with eggs, and all signs of the cihary ridges disappear ; 

 only in the terminal lacunae, in which the eggs are not found, do the ciliary ridges become 

 apparent (57 : 11, 11a). In 53: 13, is a drawing of a portion of the lateral sinus on the 

 right side of the posterior lobe of the pallium of Glottidia. For convenience of adjust- 

 ment in the plate, the lateral margin of the pallium is horizontal, the left of the fio-ure 

 being posterior. By this and other figures on this plate, it will be seen how sharply 

 defined are the sinuses and lacunae, and yet there are apparently no walls in these 



