30 DISCOPHOR.E. Part III. 



youngest or third group. Fir/. 15 of PI. X^., altliougli two of the tentacles of the 

 third group are not developed, will illustrate these relations, as the left side is perfect: 

 those tentacles marked 1 belong to the first set, those marked 2 to the second 

 set, and those marked 3 to the third set. In an undeveloped individual, repre- 

 sented by Fig. 1.3, the relative age of the tentacles is doubly set forth: in the 

 first place by the projections, and, secondly, by the difference in the size of the 

 tentacles themselves. In those exceptional cases with twenty tentacles, but which 

 originally have five, there are five corresj^onding internal projections, instead of four, 

 one being opposite each of the five primary tentacles. 



This terminates the descrii^tion of the scyphostoma period, as far as the zoological 

 characters are concerned. But before we proceed to the strobila stage, we will 

 return to the beginning and trace the histological development of the scyphostoma. 



Histology of the Scyphostoma. From the earliest period, immediately after the 

 segmentation of the yolk, to the time when the first four tentacles begin to 

 develop (PL X. Figs. 3-14, and PI. X*. Figs. 25-36), the peripheric part of the embryo, 

 whether it be an indistinct layer or has become separated from the interior as 

 a well-defined wall, consists of a mass of irregularly polyhedral cells, Avhich embrace 

 perfectl}^ homogeneous contents, and, except in the four-armed stage, bear vibratile 

 cilia on their outer surfaces. Those cells which enter into the composition of the 

 youngest embryos (PI. X. Fig. 3) differ from those of later planula stages {Fig. 10'') 

 only in being not quite so transparent, and from those of the incipiently foiu'- 

 armed stage in that a part of the latter (PI. X^ Fig. 8) are lasso-cells which are 

 scattered all over the body and crowded upon the tips of the tentacles {Fig. 7). 

 On account of the opacity of the cells of the periphery, we were unable to discover 

 by actual inspection Avhat is the nature of the mass of the cells within the body 

 of the youngest embryos (PI. X. Fig. 3) ; but when, in later stages, we had an 

 opportunity of looking into the mouth {Figs. 14" and 14") of a four-armed individual, 

 and found that the cells of the interior were identical Avith those of the exterioi', 

 we concluded that, like the peripheric cells, they had not changed from their 

 earliest condition. Not till the last of the aforementioned stages do the cells of 

 the periphery undergo any changes in their relative positions, and then they are 

 rearranged so as to form a single layer (PI. X. Fig. 14 a), excepting in the tenta- 

 cles {e), where they are replaced by a single layer of lasso-cells (PI. X*. Fig. 7). 

 The greater part of the outer waU of the tentacles is made up of a mass of 

 unchanged, irregularly polyhedral cells, but they are confined to the interior by 

 the coating of lasso-ceUs. 



By the time that the first four tentacles have become highly developed (PI. 

 X. Figs. 19, 20, and 21, etc.) and the second set of four is about budding, the 

 cells of the outer and inner walls have undergone great changes. The outer Avail 



