THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 



129 



pluteus has still a rounded posterior pole, but the arms are longer 

 in proportion to the depth of the body. About this time the invagina- 

 tion which will form the oral disc of the future Echinus appears. At 

 the end of about a fortnight the fourth pair of arms is formed, and 

 about this time the posterior pole begins to flatten until it assumes 

 the appearance of Figure 1. In about three weeks the posterior 

 epaulettes are well advanced, and the pedicellariae have appeared. 



It may be here stated that the times given for the appearance of 

 different organs are only very approximate. There is the widest 

 difference in rate of development not only between the individuals in 

 each jar, but between the larvse of separate cultures reared from ova 

 and sperm of the same Echini under apparently the same conditions. 



EXTERNAL CHARACTERS OF LATE LARVA (FiG. 1). 



^ o^wWA't'^A^ 



cL 



VyyJ'-'^' 



wr^-^ ^'.u^S 



^jJXcKJ^^^ 



Fig. 1.— Larva oi E. csculentusi x E. escidcntiij>9 . Dorsal view, x 36. 22 days old. 

 The specimen is tilted forward so that the posterior surface is shown. 

 a. cil. ep. — Anterior ciliated epaulette. al. a. — Antero-lateral arm. 

 ech. r. — Echinus rudiment. I. ped, — Lateral pedicellaria. ^;ra. a. — Pre- 

 oral arm. p. cil. ep. — Posterior ciliated epaulette, p. ped. — Posterior 

 pedicellaria. y;rf. a. — Postero-dorsal arm. po. «.— Post-oral arm. s. — 

 Stomach. 



The body is rather deeper than wide, and the posterior pole some- 

 what flattened. It is studded with pigment spots of various shades 

 from yellow to reddish brown, and of various shapes and sizes. In the 

 specimen from which the drawing was made the body was regularly 

 pigmented, while little, if any, pigment appears on the arms except at 

 the extremities. Pigment, however, varies widely among individuals 

 of the same culture in depth of colour and distribution, and this seems 

 natural, since MacBride (11) has shown that the pigment is contained 



