32 DISCOPHOR^. Part III. 



wall. There is auotlier peculiarity of the lasso-cells, which has not been noticed 

 hitherto ; we refer to the variation in their size, according to whether the tentacle 

 is extended or contracted. When the tentacle is stretched to the utmost (PI. X\ 

 Fig. 1), the lasso-cells are much smaller than those on the rest of the body, for 

 instance around the mouth (c) ; but, when the tentacle is retracted, they expand 

 {Fi(i. 3 a^ a-) to their full size, so that the wall in which they are situated becomes 

 much thicker than in the extended state, in fact as thick as the corresponding 

 wall {^Fig. 4" «) of the lower part of the body. The cells of the inner wall (PI. 

 X^ Figs. 1, 3, 4", and [> b P) have not appreciably changed since the last stage. 

 In the base of the body {Fig. A" h) they form a solid core, and are arranged so 

 that their longer axes radiate from the centre outwardly. At the base of the 

 tentacles, esj)ecially when they are retracted {Fig. 3), these cells {h^) are likewise 

 convergent toward the median line, but a prolongation of the cavity of the body 

 bounds their inner ends. The cells which form the buttress-like projections {Fig. 

 5 h~) differ in no wise from the other members of this wall. They are arranged 

 in two rows, as if they were centripetal prolongations of the doul)le wall at the 

 base of the tentacles, and form a solid column, which extends for a very short 

 distance toward the base of the scyphostoma. The structure of the sheath {Fig. 4" 

 /) has already been described in detail in a former paragraph. 



The Strobila^ of Aurelia flavidula. The first change that may be recognized 

 in the scyphostoma after it has completed its cycle of tentacles is the occurrence 

 of a well-marked constriction (PI. XP. Fig. 10 g) immediately below the outer 

 base of the tentacles. The constriction deepens until it extends at least half 

 Avay to the centre, and perhaps further, when another constriction {Fig. 11 g^) 

 appears, below the first, at a distance about equal to the combined thickness of 

 the walls of the body. This deepens until it extends as far inwardly as the first, 

 and then a third {Fig. 13 g"^) constriction divides off a tliird disk-shaped portion 

 (5). The uppermost segment {1) which bears the tentacles does not undergo any 

 change ; but by the time the third constriction {g^) has developed to the same 

 extent as the first and second, the second {2) and third (3) disks have become 

 sinuate or lobed on the ujjper edge. The lobes (/) of the second disk {2) are 

 more prominent thair those (y^) of the third or younger disk {3). There are 

 eight lobes, arranged at equal distances around the disk, and as many sinuses {i), 

 of the same breadth as the lobes. The entire circuit of the edge is slightly raised, 

 so as to give the disk a saucer-shaped figure. The lower side of the disk is also 

 wavy, or rather ribbed, and the ribs, corresponding to the lobes, converge toward 

 the centre. 



* See Vol. III. p. 80 for the meaning of tlie word Strobila as used here. 



