ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. ^79 



independent Y-shaped piece, lying dorsally to the lesophagus, although the eighth 

 pair of arms has not yet grown out. The aboral ends of the body-rods have now 

 become fully developed, being spinous and curved inwards. 



At the next stage the pluteus possesses all of its eight arms, although the preoral 

 are as yet short. This stage is marked by the appearance of the so-called Echimis- 

 rudiment. An invagination, which will become the oral disc of the future Echinus, 

 grows inwards from the exterior to meet the left hydrocrele. At this period, too, 

 the "anterior ciliated epaulettes" are formed (Plate 19, figs. 5 and 6). Ventrally, 

 between the bases of the postoral arms, and dorsally, between the antero-lateral 

 and the postero-dorsal arms, the ciliated band becomes thickened. These two 

 crescentic pairs of thickenings, on which the pigment spots are crowded, become 

 abstricted oif from the main ciliated band. They grow to form a ventral and a 

 dorsal pair of strongly ciliated bands, extending equatorially round the larva and 

 called the anterior ciliated epaulettes. At a later stage the two ventral and the 

 two dorsal epaulettes join to form one continuous ventral and one dorsal band. 

 Up to this point the locomotory organ of the larva has been the main ciliated band, 

 which extends out to the ends of the arms. After this, however, the strong cilia 

 on the epaulettes take over the function of propelling the larva, which has increased 

 greatly in size and weight. 



At the time when the anterior epaulettes are being formed, the most aboral part 

 of the main ciliated band, lying between the postoral and postero-dorsal arms on 

 either side of the larva, becomes thickened. These thickenings become abstricted 

 in the same way as those which formed the anterior epaulettes. They give rise to 

 a pair of posterior epaulettes (Plate 19, figs. 8 and 9), one on each side of the 

 larva, aiid having the same function as the anterior epaulettes. Eventually they 

 extend dorsally and ventrally round the body of the larva. Their effect is to change 

 the posterior pole firom a domed to a rather flat shape. Like the anterior epaulettes, 

 the posterior ones are strongly pigmented, although the amount of this varies greatly 

 in individuals. 



The subsequent development of the pluteus consists chiefly in the growth of the 

 jE'cAwMS-rudiment and the internal changes connected therewith. A pair of 

 pedicellaiise make their appearance, lying dorsally and ventrally on the right side 

 of the larva, opposite to the Echinus-rudinient. Another pedicellaria is almost 

 always developed at the posterior pole of the larva (Plate 19, figs. 8 and 9), and 

 sometimes two are found close together in this position. 



With the growth of the Echinus -rudiuient the arms are gradually absorbed, their 

 skeletal supports being frequently seen to project terminally. There is a stage when 

 the larva is still actively swimming by means of the strong cilia on the epaulettes, 

 while the tube-feet of the Echinus are projecting from the left side of the body. 

 By this time the larva has sunk down from the surface of the water to the bottom 

 of the culture jar. In healthy individuals metamorphosis, consisting of the absorption 



