AND ITS BIOLOGICAL SIGNIFICANCE. 325 



middle of the lorica or farther. This area shows the most varied differentiations 

 in different species, in some appearing as a single high ridge, in others as two ridges, 

 in still others as a depression, while in yet other cases there is no change in the 

 surface of the lorica in this region except the transverse striations. 



This area is unsymmetrical in position, beginning at the anterior margin at the 

 right of the middle line, and passing obliquely backward and toward the left side. 

 Its sides are, as a rule, rather sharply defined, frequently appearing as two slight 

 thickenings or ridges. 



This striated area is really a flexible portion of the lorica, and the transverse 

 striations are muscle-fibres, by means of which the edges of the area can be brought 

 closer together, thus permitting an increase or decrease in the circumference of the 

 body. The fibres are attached at the middle and at the two sides of the area (Fig. E). 



In different species of the Rattulidse we find the following different conditions 

 of this area: (1) in a very few species both edges of this area are elevated into ridges, 

 leaving a deep groove between them (Fig. E, 7) ; (2) in some species the median part 

 of the area is depressed, so that a shallow groove is formed; (3) in other cases the 

 area lies at the general level of the lorica; (4) in the condition which may be considered 

 typical for the Rattulidse, and which is found in the greatest number of species, the 

 right edge of the area is elevated to form a high ridge, while the left is not elevated 

 at all. Thus, in these typical cases, shown in Diurella tigris Muller (Fig. E, 1), Rat- 

 tulus carinatus Lam. (Fig. E, 9), and Rattulus lophoessus Gosse (Fig. E, 10), there 

 is a high oblique ridge passing from the right side at the anterior end backward and 

 to the left, and sloping strongly to the right. The edge of the ridge is thus directed 

 toward the right, and the right side is either perpendicular or concave. Why the 

 right side of the striated area should have thus developed rather than the left will 

 be brought out in our general discussion of asymmetry. 



A word further should be said as to the relation of the striated area to the teeth 

 or spines at the anterior margin of the lorica. These are formed as outgrowths of 

 one or both of the thickened edges of the striated area. Where two teeth are present 

 both edges project, that formed by the right edge being usually the larger. When 

 only one tooth is present it is formed by a projection of the right edge of the area. 



5. Foot. — The foot is a short conical structure attached to the body at its pos- 

 terior end (Fig. E, 1, pd.). The attachment to the body is usually unsymmetrical, 

 the joint between the foot and body being generally oblique, extending farther back 

 on the left (or left dorsal) side than to the right. This is well shown in Figure E, 6, 

 of Rattulus elongatus Gosse. In some cases the posterior edge of the lorica projects 

 backward some distance over the foot on the left side, but not on the right (Fig. E, 10) . 



