310 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



all are pointed. The paired spines situated dorsally to each primary tube-foot have 

 increased somewhat in size, but are considerably shoi'ter than the interradial spines. 

 They never grow to any considerable size. Figs. 88 and 89 show following stages, 

 in which the individuals have increased considerably in size. In fig. 89 another pair 

 of tube-feet and two spines have been developed in each radius, and a number of 

 pedicellarife have made their appearance. As described by MacBride, the terminal 

 tube-feet eventually degenerate into the so-called eye-spots. 



The early development of the external features is similar in E. acutus to that in 

 E. esculentus. As in the latter form, there is no trace of the paired tube-feet at 

 metamorphosis (Plate 23, fig. 90). The first rudiments appear a day or two afterwards. 



Plate 23, fig. 91, shows E. miliaris immediately after metamorphosis. It will be 

 seen that there is, in each radius, a pair of functional tube-feet in addition to the 

 unpaired primary ones. These structures are indeed developed some time before 

 metamorphosis, and both the paired and the primary tube-feet can be seen projecting 

 from the ^c/mws-rudiment while the pluteus is still swimming. In regard to the 

 development of the spines and the other external features, E. miliaris is no further 

 advanced at metamorphosis than are E. esculentus or E. acutus. The distribution of 

 the spines is similar in the three species. 



The investigation into the inheritance of the tube-feet character in the hybrids 

 showed that this was not a reliable and definite feature, as were the posterior 

 ciliated epaulettes and the green pigment masses of the plutei. In the first place, 

 it is a character which is present only for a short time — at, and immediately after, 

 metamorphosis. Now it frequently happens that cultures of liybrid plutei become 

 retarded in their development and do not reach the stage of metamorphosis until 

 three or four months after fertilization. The individuals in such cultures seem to 

 experience a difiiculty in absorbing the body of the pluteus at metamorphosis, and 

 the process may take a week to complete instead of the usual few hours. In the 

 meantime the spines and tube-feet of the Echinus are continuing to develop, and, 

 when the metamorphosis is eventually complete, the hybrid may have developed 

 the paired tube-feet, not because it has inherited them, but because the meta- 

 morphosis has occupied some time. It is easy to discard such retarded cultures, but 

 in the cases when the plutei develop at the normal rate there are usually present a 

 few weakly individuals which experience a difficulty in accomplishing metamorphosis. 

 From these latter it is easy to draw a false conclusion as to the inheritance of 

 such a transitory character as that of the presence or absence of the tube-feet at 

 this stage. 



Besides this, the tube-feet seem to be subject to a certain amount of irregular 

 variation with respect to the number which are developed. No exceptions were 

 found to the rule that in pure-bred E. esculentus or E. acutus there are five tube- 

 feet at metamorphosis, while in E. miliaris there are fifteen. It was in the hybrids 



