Chap. III. GENUS PLEUROBRACHIA. 223 



about forty-five degrees, and at a right angle ; and after the same manner all the 

 cells of the lateral system, receding from tlieir peripheric terminations toward their 

 bases, cross those of the radial system. It must be borne in mind, however, that 

 these various angles of traverse are not as if formed by lines coursing on a plane, 

 but as if on the surfice of a sphere ; that is, they are spherical angles. 



This, in genei'al terms, may be said to be the relation of those cells of the 

 radial and lateral systems which lie between the equatorial plane and such a 

 plane as would divide the body by traversing it on a level with the basal ends 

 [Fitj. 2o y) of the tentacular apparatus; but beyond this zone and towards the 

 mouth, the relations of these cells change very rapidly : within the zone the cells 

 of the lateral system diverge from the tentacidar socket about at right angles to 

 its axis, but beyond this they diverge at a gradually lessening angle, till at the 

 tip of the socket {y) they trend, as it Avere in direct continuation of its axis, to 

 the nearest point in the periphery, somewhat in the same manner as the hairs 

 project from the tail of a squirrel along the sides and at the end. As the cells 

 of the radial system do not penetrate the s]3aces which intervene between the 

 tentacular sockets and the interambulacral bands, which the plane of the former 

 bisects, the cells of the lateral system are here left free to act b}^ themselves. 



The motory sj-stem of the tentacles is so intimately interwoven Avith their Avhole 

 structure, that it is most con\'enieut to describe it when presenting the anatomy 

 of these organs in detail ; but we will make one or tAvo remarks in this connection 

 in regard to the relations AA'hich their different Avails bear to those of the body. 

 Although the outer waU {Fie/. 15/3 (i' (i", and p. 235 Fiff. 87 ji" (i'") of the tenta- 

 cular base is composed of much smaller and differently shaped cells from those of 

 the sockets (/ j'^ y), yet Ave must believe that the Avails of the tAvo are essentially 

 one continuous layer; and, referring to A\-hat we have pre\'iously A'entured to sug- 

 gest, that these sockets are depressions in the interambulacral layer, and also that 

 the AA'hole tentacular a2)paratus is a prolongation of tAvo 02:)posite jioints of the 

 peripheric .system, endowed with the fliculty of more extensiA'e motion than the basis 

 from Avhich it arises. Any one ftrmihar with the very suujjle tentacular ap^jai'atus 

 of Bolina, Chiaja, LeSueuria, and Euranipha?a, Avill readily comprehend that Avhilst in 

 them the tentacular sockets are shallow dej^ressions from Avhich the peripheric 

 prolongations arise, in Pleurobrachia these sockets differ only in degree by being 

 more deeply plunged into the mass of the body. The inner wall {Firj. 15 (f and 

 Fifj. 87 7 J'') of the tentacle (Jc) and its base {fj /), although very thick at the 

 latter point, is, to aU ajipearance, identical in cellular structure and continuous 

 Avith the thin Avail {Fig. 87 ;'" /'"' e) of the chymiferous tubes, Avhich Ave have 

 next to describe. Throughout the Avhole extent of the digestiA'e caA'ity and chy- 

 miferous system, the wall is composed of extremely elongated cells, which, trending 



