324 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



internal organs comprise an alimentary canal, nervous, muscular, excretory, and 

 reproductive organs, together with certain mucous glands and reservoirs. In the fol- 

 lowing, such of these organs as are concerned in the asymmetrical structure will be 

 taken up in detail. 



1. Form. The unsymmetrical structure of these animals is at once evident in 

 a cursory examination of the body form. If we examine dorsal views of a number 

 of the Rattulidse (Fig. E), we find that all are curved, so as to be concave to the right, 

 convex to the left. The curve is often not a simple one, but is of such a nature that 

 the body forms a segment of a spiral. This is perhaps best seen in Figure E, 1, repre- 

 senting Diurella tigris Muller. This is a characteristic which it is difficult to represent 

 in a figure, though often very noticeable in the animal itself. 



2. Lorica. The lorica, or shell, covers the body completely, being without open- 

 ings at its sides, but there is an anterior opening for the protrusion of the corona, and 

 a posterior one for the protrusion of the foot. 



3. Head-sheath. The anterior part of the lorica is usually 

 set off from the remainder of the body by a slight constriction. 

 This anterior portion covering the head may be called the head- 

 sheath. The head-sheath frequently has longitudinal plaits, 

 which allow the sheath to fold when the head is retracted within 

 the lorica. 



Especially interesting from the present point of view are the 

 teeth, or spines, which are found in many species at the anterior 

 margin of the head-sheath. When present at all there are one 

 or two of these, and they lie as a rule not in the dorsal median 

 line, nor at equal distances on each side of the middle line, but 

 distinctly to the right of the dorsal line. These teeth are shown 

 in Figure E, 1-5, and in Figure F. They form prolongations of 

 the "striated area" of the lorica, described below. Where two 

 teeth are present, as in Figure E, 2, 4, 5, and in Figure F, they 

 are usually not equal, but the right one is considerably longer 

 than the left. (In Diurella stylata, Figure E, 2, however, they are 

 nearly equal.) One of the two teeth, when two are found, lies 

 l?" nearly in the middle line, while the other lies distinctly to the 



view, showing the un- right of this line. 

 equal anterior teeth 



(original). 4. Striated Area and Ridge One of the most peculiar 



characteristics of the Rattulidse is the presence on the lorica of a dorsal longitudinal 

 area, striated transversely, which extends from the anterior edge backward to the 



