27H MESSES. C. SHEAEER, W. DE MOEGAN, AND H. M. FUCHS 



however, to show that laboratory conditions have a large influence on the rate of 

 growth, and, in consequence, the times given in the following pages for the attain- 

 ment of the various stages must not be taken as invariable. 



Below is a description of the development of E. esculentiis, and, following that, 

 those points in the development of E. acutus and E. miUaris which differ from 

 E. esculentus are detailed. 



(i) The Development of E. esculentus (Plate 19, figs. 1-9). 



The gastrula (fig. 1) is an ellipsoidal organism, flattened at the pole, where the 

 invagination has taken place. The blastocoele contains scattered groups of 

 mesenchyme cells, and on either side of the invagination a group of these cells has 

 commenced to secrete the larval skeleton in the form of a tri-radiate spicule. 



The next stage (Plate 19, fig. 2) is termed the prism larva. The postoral arms 

 [the nomenclature used in this paper is that of Mortensen (68)] have appeared 

 antero-ventrally and the oral lobe has developed antero-dorsally, but as yet there 

 is scarcely any indication of the antero-lateral arms, which will grow out from the 

 oral lobe. The main parts of the larval skeleton can now be distinguished. There 

 is a main body-rod on either side extending to the aboral pole of the larva. These 

 body-rods are continued anteriorly as the supports of the postoral arms. In Echinus 

 the postoral rods are single. From the point where the postoral rod passes into the 

 body-rod on either side spring the supports of the antero-lateral arms, and also a 

 pair of ventral cross-pieces which will eventually meet in the ventral middle line. 

 Over the surface of the larva are scattered pigment spots of a yellowish -brown colour. 

 At all larval stages the amount and the tint of the pigment varies widely, even in 

 individuals of the same culture. The aboral pole of the prism larva varies in form, 

 being sometimes flattened, at other times domed as in the succeeding or four-armed 

 pluteus stage. 



At the next stage (Plate 19, fig. 3) the antero-lateral arms have commenced to 

 grow out. The ciliated band can be seen round the anterior end of the larva, 

 extending out to the ends of each of the arms. The skeleton has increased in size, 

 and a change has appeared in the aboral ends of the body-rods, which are becoming 

 spinous and are curving in towards one another. This will become more marked at 

 a later stage. The aboral pole of the larva is dome-shaped. The mouth can be seen 

 at the base of the oral lobe. It leads through a short oesophagus into the large 

 stomach. The anus is in the mid-ventral line. The ccelomic pouches can be seen 

 at the sides of the oesophagus. The pigment is becoming concentrated at the ends 

 of the arms. 



The succeeding stage (Plate 19, fig. 4) is marked by the commencement of the 

 postero- dorsal arms, which are outgrowths from the lateral curves of the ciliated 

 band. Each is supported by a skeletal rod, which is developed independently of the 

 rest of the skeleton. The skeleton of the preoral arms has apjjeared as an 



