KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 21 



Only one among' them wus entire, but taken all together they aft'orded materials enough 

 for observing some details not noticed before, PL XIV, fig. 163 — 171. Although ap- 

 parently not far from leaving the nursery, they are still without oesophageal and ex- 

 cretory openings. In length they measure 2,3 mm., in breadth 1,9 mm. The general 

 form of their test is very much that of the parent, as is also the dark brown colour. 

 The whole body, spines and all, are tightly enveloped by a thick membraneous cover- 

 ing, the larval covering, closed at every point, and devoid of any opening for the 

 alimentary organs, the animal still subsisting solely on the nutritious matter in store 

 from the previous state of its development. In this stage now, the last one of its 

 evolutional life, a stage of external rest passed through alike by the Cidaridaj and 

 the Echinidaj, and by the Spatangidie, the future skeleton is being built up, while in- 

 ternally the different organs receive their final structure as demanded by the mode of 

 life of the adult. Under the thick envelope of the young Abatus cavernosus the spines, 

 straight, conic, ei'ect and bristling, look strong like those of an Archaaonomous Echinoid, 

 while the calcified plates of the ambulacral and interradial systems are already dis- 

 tinctly visible, and can be followed all the way up to the calycinal system, fig. 163, 166, 

 167. The skeleton, thus laid out in its principal parts, presents two clear spaces, 

 both however overspread by the general covering and both unpierced, the one on the 

 ventral side, not much before the middle, being the pentagonal stoma, fig. 16.3, 165, 

 the other, on the dorsal side, a little behind the middle, being the central part over 

 which the calycinal system is forming, fig. 164, 164 A. When the test is cut open 

 horizontally, the intestinal tract is seen to be thick in its middle part, contracted 

 in its oesophageal and rectal parts. Both poi^tions end cfecally, and their closed ex- 

 tremities are seen to touch the wall of the visceral cavity, to which they are attached 

 by the peritoneal lining descending upon them: the oesophageal extremity at the centre 

 of the stoma, fig. 165, 167 , the excretory at the centre of the clear space of the back,/^. 

 166, 168. Thus, at this stage, while directing its oral end towards a point slightly in 

 front of the middle of the ventral surface, and its excretorial end to the central part 

 of the calycinal region, only a little behind the middle of the back, the intestinal tract 

 for a while has an ortho-proctic position, and recalls its Endocyclic termination in the 

 ancient types. But this is here only a transitory condition, the centre of the caly- 

 cinal system remains unpierced, and the excretory end of the intestinal tube, still 

 blind, retrogrades, and opens only when beyond the limits of the calycinal system, — 

 a movement, analogous to that known to have taken place, during past geological 

 periods, through a long succession of Exocyclic species. The two large plates of the 

 interradium 5, fig. 166, that approach nearest to the calycinal system, are those of the 

 sixth pair, the labrum counting as the first. And, because in the adult the periproct 

 is surrounded by the fifth, sixth and partly the seventh pairs, and, as I have shown in 

 the case of Brissopsis lyrifera '), the periproctal plates remain the same in the adult as 

 in a specimen not very much older than this young one of Abatus, it may be presumed 

 that, in consequence of the forward movenjent of the apical part of the test, and 



>) Etudes p. 60, pi. XXXVII, fig. 218. 



