L U C E K N A 11 1 .1-: A N D T II E I K A L L I E S . 81 



§ ^S. A Young Specimen ^ of an inch across. Special Development of a 7'entacle, a 

 CoUetocysiophore, and a Genital Sac. (PI. iii, /i(/s. 29-3:3; I'l. v, Jif/s. 58, 59; 

 PI. \n,Ji(js. G7-73.) 



161. This specimen exceeds the last more in size than in tlic degree of develop- 

 ment of its organs, and we might, therefore, ahnost say tliat the following descrip- 

 tion is but a continuation of the previous one. Altliough only one-eighth of an 

 inch across, the umhella seems broader because the tentacles are so conspicuous, 

 and are longer and more slender than in the last stage. This will appear plain 

 enough if we reflect that as the longest tentacles {Jitj. 33) are one-fortieth of an 

 inch long, they extend the limits of the body one-twentieth (j%); and as the 

 umbella is one-eighth (/,j) of an inch across, the proportion of (lie latter to the 

 augmentation by the tentacles will be as /'^ to ^^.^ oi- 5 to 2. In the full grown 

 animal the proportion is as 4: to 2. 



lt)2. The Tentacles. — We have recorded, above, the relative length of the oldest 

 tentacles of this specimen; and now propose to trace the development of these 

 organs from their initial stage up to their full stature. The foundation of this 

 investigation lies in the specimen before us, but the older stages are to be sought 

 for in advanced animals, where the multiplicity of tentacles furnishes a more com- 

 plete series than where there are only ten or a dozen of these organs in a group. 

 In the first place we will recall the fact, already stated (^ 93), that the youngest 

 tentacles are always found on the proximal side of a group, and that the oldest lie 

 next the boundary line between the circumoral and aboral faces of the umbella. 

 The earliest trace of an incipient tentacle is exhibited by the appearance of nume- 

 rous nematoci/sfs {Jiy. 58, ^-), collected in a closely crowded group very near the 

 bases of the other tentacles. It is, therefore, evident that the globular, nemato- 

 cystigerous tip, the prehensile region, jj«r excellence, is the first to develop, and, as 

 we shall see hereafter, to perform the duties assigned to the organ of which it 

 forms a part. The shaft is merely the basis, the handle as it were, by which the 

 instrument is moved from place to place. After this a rounded protuberance (///. 

 59, ip-) rises gradually above the circumjacent area, bearing on its summit the 

 group of nematocysts just mentioned. A profile view of this bud discloses its rela- 

 tion to the walls of the body, and we find that from the beginning the whole depth 

 of the circumoral face (0 is enlisted in the process, even to the innermost wall, the 

 gastrophmgma (i) ; for as tlie bud rises on the outer surface tlie imier wall also 

 pushes outward with the others, and thus a hollow hernia is formed by the com- 

 bined action of all. The elements concerned in this process are as follows, taking 

 them in tlieir order from without inward: first, the npsoph ra<jma (n), in which the 

 nematocysts are imbedded; second, the opsnmi/oplax (m); third, a faint trace of 

 the chondromyoplax (h); and fourth, th(> gasfrophragma (i). The first, at the out- 

 set, is considerably thicker than in the adjacent parts. Tlie s(>cond offers notliing 

 remarkable. The third, from its peculiar relation to the second (see % 65), is not to 

 be separated from it by any trencliant line. The fourth is thicker than its honio- 

 logue (i) in the unilx-lla, but thinner than in the older tentacles next to it. From 



11 February, 1878. 



