CTENOPHORA 145 



apparatus, a pear-shaped or heart- shaped form is evolved 

 (Fig. 70 .4). 



Hand in hand with these changes goes the differentiation 

 of the ectodermal structures which are characteristic of the 

 Ctenophora : the tentacular apparatus, the ciliary plates, 

 and the apical sensory organ. At an early period are to be 

 noticed in the upper'half of the body two ectodermal thicken- 

 ings (Fig. 68 t) lying opposite to each other in the trans- 

 verse plane ; such an abundant multiplication of ectoderm 

 cells occurs in these places, that they become many layers 

 deep. These two thickened areas form each the fundament 

 of a so-called tentacle base (Fig. 69 B, th). Within the 

 territory of these a ridge, known as the tentacle stalk 

 (Fig 69 B, ts), soon makes its appearance, and out of it 

 arises the fundament of the tentacle (t). At the same time 

 with the earliest formation of the tentacular apparatus, four 

 rows of cells situated adradially become conspicuous by 

 their active proliferation. These cells are covered with 

 cilia, which at first are short and fine; they begin to beat 

 slowly backwards and forwards, and soon fuse together in 

 such a way as to form the swimming plates (Fig. 68 r). In 

 this way two rows of swimming plates arise on each of the 

 four fundaments, so that in these first stag-es the eisrht 

 ciliated ribs appear grouped in pairs. Originally each rib 

 exhibits only a very few (usually four to six) swimming 

 plates, and, as a rule, their number is not increased until 

 after the abandonment of the egg-membranes. The swim- 

 ming plates, as has been shown, are to be regarded as fused 

 cilia ; they are higher differentiations of a continuous coat 

 of cilia attributable to the ancestors of the Ctenophora. 

 In this connection it is interesting to know that Chun 

 (No. 3) was able to demonstrate on the embryo of 

 Encharis multicornis a fine ciliation covering the whole 

 surface. Of this ciliation there are retained throughout life 

 only eight fine meridional rows, which extend from the 

 rows of swimming plates to the upper pole of the body, and 

 establish the connection with the sensory body located 

 there. This apical sensory organ, which is pei'haps to be 

 considered as the centre of the nervous system, is also de- 



K. H. E. L 



