280 MESSES. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



of the body of the larva, is accomplished in the space of a few hours. In the case of 

 a weakly larva, especially if the development has been very slow, a week may be 

 taken to complete the process. In general, however, the quicker the metamorphosis 

 the more healthy and vigorous the sea-urchin produced. 



(ii) The Development q/E. acutus (Plate 19, figs. 10-18). 



The question of the relationship of E. esculctdus and E. acutus has been fully 

 discussed in a previous section. At all events these forms are closely related to one 

 another, and a corresponding similarity is found between the larvae. In form the 

 early larva of E. acutus is similar to that of E. esculentus. The pigment is rather" 

 more in the form of dots and stippled lines, but as usual this is very variable. In 

 the late pluteus the pigment is usually darker and moi-e abundant than in 

 E. esculentus. 



The most pronounced difference between the plutei of E. acutus and E. esculentus 

 is in the skeleton of the early larva. In E. acutus the aboral ends of the body -rods 

 are more robust and more spinous than in E. esculentus, and moreover, they do not 

 show so much bending in towards the middle line. This matter will be more fully 

 dealt with in the section on the larval skeleton. 



The fully-formed pluteus (Plate 19, fig. 18) has a smaller body and more slender 

 and divergent arms than E. esculentus, but resembles the latter in all essential 

 features. The development of the epaulettes and of the pedicellarise is the same. 

 Small differences in general shape are, however, very inconstant, and no great 

 importance can be attached to them. 



(iii) The Development q/'E. miliaris (Plate 19, figs. 19-25). 



E. miliaris is a form less closely related to E. esculentus and E. acutus than the 

 latter are to one another, and its larva shows important differences. 



During the seasons of 1909-11 we always found that E. miliaris was by far the 

 easiest form to rear, and that its development was the most rapid. This, we thought, 

 was probably connected with the fact that E. miliaris has its habitat on the shore, 

 and in consequence laboratory conditions should suit it better than the other forms, 

 which live in deeper water. In 1912, however, the case was exactly the reverse. 

 Although fertilizations were as easy to make as previously, yet it was only with 

 great difliculty that healthy larvae could be obtained in a late stage. Indeed, pure 

 E. m,iliaris was more diflicult to raise than any of the hybrid crosses. 



The egg of E. miliaris is considerably smaller than that of E. esculentus, and in 

 consequence the early larva is smaller. It difiers, too, in general form (Plate 19, 

 figs. 20-21). The arms are shoi'ter in comparison with the body, and the aboral end 

 is more pointed, resembling in side view a conical cap. There is less pigment than 

 in E. esculentus, and it is distributed in small dots. A well-marked preoral lobe 

 overhangs the mouth, and the larva is distinguished by a glassy transparency. The 



