Chap. III. GENUS PLEUROBRACIIIA. 221 



Next to the radial system, the lateral s//sfcm {Fig. 21 ;? j?, and Fig. 23 j>^) is 

 the largest, and, of the two, the most curiously arranged. The tentacular appa- 

 ratus {h^ /r) may be said to be the basis of this system; at least, from these two 

 points all its cells radiate to the periphery. In a view from the oral end of 

 the body [Fig. 21), each half (pf) of the system presents an outline which reminds 

 one of the wings of a butterfly, the tentacular apparatus simulating the body of 

 the insect : nowhere do we find the cells trending in straight lines, but always 

 in gentle curves, whether it be toward the oral plane or in the tentacular plane, 

 or to all intermediate points of the periphery; or whether, as a profile view shows 

 {Ft'g. 2.3), toward the oral area, or in the opposite direction toward the orifice of 

 the tentacular sockets, or to all intermediate points. In order to simplify the 

 description as much as possible, we wiU speak of the curved rows of cells as of 

 the cells themselves, which is the more appropriate since the long curves are made 

 by adding one curved cell to the end of another. In the first place we would 

 mention the important fact, that the tentacular sockets (,/) alone form the basis of 

 this system, and that the tentacular apparatus proper {/i^ Ir) has no connection 

 whatever Avith it. As we see it from the oral end {Fig. 21), the inner and proxi- 

 mal face {]/) of each of the four Avings of this system forms the hypothenuse 

 of a rightrangled triangle, of which the oral and tentacular planes constitute the 

 other two sides. Properly speaking, this fiice is the hypothenuse of a sjiherical 

 triangle, since it makes one continuous curve from the edge of the tentacular 

 socket (/) to the median line of that peripheric l)aud (A, E) which is bisected by 

 the oral plane, and meets its corresponding hypothenuse from the opposite side, at 

 a very acute angle, all along this line, to the end {it') of the band, and then the 

 edge of this face diverges from the oral plane and follows the outlines of the oral 

 system {iii^) to the tentacular plane. It will be readily inferred, that the greatest 

 span of this face is on a level with the equatorial plane of the body, and that 

 it gradually shortens toward the oral region, and also in tlie opposite direction. 

 If we follow it along its attachment to the tentacular socket toward the oral area, 

 we find that it meets the face of the wing on the other side at a short distance 

 from the bottom of the socket ; so that the two faces form one continuous surface, 

 whicli arches, as it were, over that portion (Fig. 23 m^) of the radial system that 

 lies in the tentacular plane. The Ijottom {j^) of the tentacular socket projects 

 consideraljly beyond the base of the tentacular apparatus (A^ /i^), and is free from 

 it ; and it is where the wall of this part of the socket joins the terminal edge of 

 the tentacular l)ase that the margins of the above-mentioned f;xces come together. 

 If, now, in looking at this fiice in profile {Fig. 21 p^), — as we may do by viewing 

 it from the oral end of the body, — we follow it with the eye from the equatorial 

 region toward the oral area, its component cells gradually change their trend, in 



