STRUCTURE AND DEVELOPMENT OF COEl,OP],ANA. 51 



tentacular rudiments become more and more prominent, and raised upon 

 the surface of tlie gastrula on each side into a narrow longitudinal 

 ridge-like elevation (figs. 22 & 23). The ribs develop comb-plates, of 

 which there appear at first six or seven in each rib nearly at the same 

 time. The cilia of the comb-plates are very fine and short as yet and 

 are not easy to observe. The stomodaeum shows itself now as an 

 invagination having a distinct internal cavity. 



The thickenings of the rudiments of the tentacles gradually disap- 

 pear from the surface of the body, by sinking down below the level of 

 the surface (figs. 24, 25). Among the endodermal cells, appears a space 

 communicating with the broadest part of the axial cavity mentioned 

 above and shaped H in the dorsi-ventral view, with the transverse bar 

 of H situated in the pharyngeal plane and the two lateral bars parallel 

 with the tentacular plane, each of the apical and basal ends of the latter 

 bars lying at the centre of each body quarter formed by the intersection 

 of those two planes. 



Meanwhile, the stomodaeal invagination elongates up to almost 

 the middle point of the vertical axis (figs. 26 & 28). The broadest part 

 of the axial cavity of the endoderm is carried upwards, together with 

 the ascending apex of the invagination, to a level somewhat nearer the 

 aboral than the oral pole. Subsequently, the cavity is widened into the 

 lumen of the infundibulum, while its branches become the endodermal 

 sacs of the embryo. 



The gastrula now has become a typical cydippid larva. At first, 

 the surface of the body of the embryo appears uneven, the outline of 

 each ectodermal cell being shown quite distinctly; soon, however, it 

 becomes smooth all over the surface. The combs begin to beat, at 

 first each in its own rhythm, but afterwards in unison, so that the 

 embryo commences to rotate in the egg-membrane. 



Cydippid Larva. 



PI. 7, figs. 1-3 represent an unhatched cydippid larva of a very 

 early stage. The shape of the body is spheroidal narrowing gently 

 towards the oral pole and with its pharyngeal and tentacular axes 

 nearly equal and slightly longer than the vertical axis. At the aboral 

 end of the last mentioned axis, lies the sense-organ {s) and at the 

 opposite end, the mouth (?«), besides a pair of tentacular rudiments (/) 

 and eight ribs {c) both located in the aboral half of the body. The 



