1907.] NATURAL SCIENCES OF PHILADELPHIA. 87 



on the ventral surface. One of the hitter, the preoral tract, clothes 

 the ventral surface of the head, and at its posterior margin passes into 

 the ciliated lining of the mouth. It serves the obvious purpose of 

 creating currents of water which sweep particles of food toward and 

 into the mouth. The second or postoral tract comprises a narrow 

 strip beginning at the posterior Ijordcr of the mouth and continued 

 backward to the end of the caudal appendage. This strip is widest in 

 its anterior portion, having here a width of approximately one-half of 

 the diameter of the ])ody. From this point it narrows posteriorly and 

 is reduced to a third of its original width on reaching the caudal 

 appendage. 



The ciliated rings number nine; two belonging to the head and seven 

 to the trunk. The two cephalic l^ands are not, however, continuous, 

 each being interrupted on the middorsal surface of the head by a con- 

 siderable gap; both of the two rings are thus divided, with the aid of 

 the ventral ciliated tract, into two lateral halves. The two bands com- 

 posing the first ring arise on the ventral surface, at the lateral edges of 

 the ciliated tract, a short distance posterior to the anterior pole of the 

 head; they then pass upwards and backwards in a gentle curve, and 

 terminate between the eyes (see text fig. I). These bands are narrow, 

 and composed of but few rows of cilia. The two bands representing 

 the second cephalic ring embrace the head at its greatest diameter, 

 and lie in a plane at right angles to the long axis of the animal. These 

 bands are relatively broad and composed of numerous rows of long 

 cilia. As is the case with all the ciliated rings the cilia beat rhythmically 

 backwards, and are only clearly seen in their forward position, that of 

 recoil. 



In the fh'st preoral or cephalic band of D. vorticoides, as described by 

 Schimkewitsch (1895), there is a similar gap. It is possible that in 

 other species of Dinophihis the cephalic bands are also not continuous 

 dorsally, since the cilia are here not easy to make out, except when 

 the animal is seen in profile, or by the study of sections of fixed and 

 stained material. In the case of this species I was at fii'st misled as 

 to the course of the cephalic bands aRd figured them incorrectly (1904a, 

 text fig. VI). This misapprehension was due to the figiu-es and 

 descriptions of other species, and also to the presence of sensory cilia 

 on the dorsal surface of the head, as described below. 



The discovery of the presence of a dorsal gap in the second cephalic 

 ring was of particular interest to me, inasmuch as it tends strongly to 

 confirm my conclusions as to the origin of this band and its homologj'- 

 with the prototroch of the trochophore larva, as expressed in 1904. 



