ECHINODERMATA 439 



the sea-urchin are as follows: inside the Pluteus oi Strongylo- 

 centrotus lividus, which is provided with four arms, we tind 

 nearly the same conditions that have been described apropos 

 of the development of the enterocoele and hydrocoele. The 

 enterocoelic sacs lie to the right and left of the stomach ; the 

 hjdroecele lies over the left one of these, and has the form 

 of a retort, the neck of which opens to the exterior on the 

 back of the larva, somewhat as in Figs. 212 and 218, 

 figures of a Spatangoid ; the conditions in these forms are, 

 however, somewhat different, as will be mentioned farther 

 on. Later, when the Pluteus has become six-armed, an 

 invagination of the outer skin is formed over the hydrocoele 

 (Fig. 212). This arises from a thickening of the epidermis, 

 which gradually sinks in and finally rests with its bottom 

 upon the hydrocoele.^ The thickened discoid bottom of the 

 dermal invagination is the earliest fundament of the lower 

 (oral) surface of the body of the sea-urchin (called " Echinoid 

 disc" by Joh. Muller). The much thinner lateral parts of 

 the invagination overlie this as an amnion-like covering 

 (Fig. 213). The opening of the invagination has narrowed, 

 but persists, whereas in the Spatangoids other conditions 

 subsequently make their appearance (comp. infra). The 

 hydrocoele now grows out into five processes, and the 

 Echinoid disc does the same, by developing a dermal cover- 

 ing over each of the five hydrocoele processes. In this way 

 the first five feet of the sea-urchin arise. They extend into 

 the cavity of the invagination, almost filling it. 



During the changes described in the region of the Echinoid 

 disc, the first indications of the dorsal surface of the sea- 

 urchin also become noticeable. Two roundish dermal eleva- 



i Figures which Fewkes (No. 13) gives of the developmental stages of 

 Eehinarachnius parma may confirm Metschnikoff's deBcription, although 

 this cannot be gathered from the text of the work. Likewise it seems to 

 us from the figures of Colton and Gakman (No. 11) that the metamor- 

 phosis of Arbacia is like that described by Metschnikoff for EeJdnoids 

 and Spatangoids. A cavity appears on the Pluteus, in which the first 

 formed feet become visible. The work of Coltox and O^H^l^n was un- 

 fortunately not accessible to us, and is known only through the descrip- 

 tion of Brooks [Handbook of Invertebrate Zoology, Boston, 1882). 



