ECHINOCYAMUS PUSILLUS. 49 



and gradually takes the shape of a fine membrane, PL VII, fig. 92. 

 This diflerentiation of the cells in the sac is easil}^ traced from the 

 beginning of the invagination. So far as I have been able to see, the 

 opening of the invagination becomes subsequently closed, which accor- 

 ding to Metschnikoff also takes place in the Spatangoids. I may say 

 however that I have thought I have observed in some advanced larvaj 

 an exterior opening still remaining, PI. VII., fig. 95. The thick-walled 

 bottom of the disc-lik« sac plays an important part in the development 

 of the young sea-urchin, while the rest of the sac only serves as a kind 

 of amnion. 



When the disc is in contact with or not far from the hydrocœl. 

 the latter sends out into the former five tubular processes or rather 

 feet, which thus have two different origins, the outer investment from 

 the ectoderm, i. e. the thick-walled part of the invaginated sac, and 

 the inner from the mesoderm, i. e. the hydrocœl, PL VII, fig. 92 — 96. 

 The feet arrange themselves round the periphery, leaving a central space 

 of the disc free, which gradually assumes a pentagonal form and be- 

 comes the buccal disc of the outgrowing sea-urchin. The buccal mem- 

 brane remains for a time entire and unpierced. Now also the first traces 

 of calcareous plates, spines and sphseridia make their appearance, still 

 enclosed by the membraneous amnion. They arise in the following- 

 order. First the five interradial laminse begin to calcify and immedia- 

 tely after the beginning of this process their spines begin to originate 

 from their own centres of calcification. Then five pairs of smaller 

 rac^ial calcareous plates come to view, one pair in each interstice be- 

 tween the primary plates. Shortly afterwards one or seldom two small 

 singular bodies appear to arise on the upper side of some of the radial 

 laminae, which are probably sphaîridia, and almost simultaneously the 

 first indications of the dentary apparatus become traceable, PL VIII, 

 fig. 106. Of course, during its growth the young sea-urchin has broken 

 through the amnion and the larval epidermis, so as to set the spines 

 and feet at liberty. Simultaneously the skeleton of the Pluteus has 

 been gradually undergoing atrophy and becomes so much absorbed that 

 only a few rods more or less destroyed are left, when the young sea- 

 urchin has passed the stage of Pluteus in order to settle at the bottom 

 of the sea, PL VIII, fig. 100—102. 



In the youngest stages of sea-urchin which either swam in the 

 water or crawled about on the bottom of the aquaria, I have very of- 

 ten, if not always, found rods protruding from the body, which are 



N'ova Acta Reg. Soc. Sc. Ups. Ser. III. 7 



