STUDIES ON THE DEVELOPMENT OF ECHINOIDEA, 319 
is typically developed. The larva is distorted, however, 
since the oral lobe containing stotnodasum is, as it were, 
twisted round on the body, and the result of this twist on the 
disposition of the skeletal rods is at first not a little puzzling. 
Care, however, enables us to recognise all the constituent 
parts of the maternal skeleton. 
To sum up : The hybrid produced by fertilising the eggs 
of Echinocardium cor datum with the sperm of Echinus 
esculentus follows the mother in the character and distribu- 
tion of the pigment : it is much smaller than larvae of either 
the paternal or maternal species; it almost always follows 
the father in the total absence of the aboral spike and of its 
supporting skeleton, since in only one hybrid out of the 
hundreds examined was the aboral spike formed. In 
the skeleton of the post- oral arms the hybrid may be of 
the paternal type, of the maternal type, or of an intermediate 
character. In the inbending of the aboral ends of the body 
I’ods the hybrid follows the father. 
The most important of these results is undoubtedly the 
total inhibition in the vast majority of cases of the formation 
of the aboral spike in a larva developed from a Spatangid 
egg, and the formation of a larva with a rounded aboral end 
and in-bent body rods, recalling in these features the Echinus 
larva. When we recollect that according to Shearer, 
De Morgan and Fuchs the ci’ossing of two species of the 
genus Echinus results in the production of larvae of the 
maternal type, no matter what feature is considered, it is not 
a little remarkable to find the paternal influence so strong in 
a cross between two species belonging not only to different 
genera but to different orders, species which must have 
diverged from a common ancestor at the beginning of the 
secondary e[)och many millions of years ago. 
If the attempt be made to explain the absence of the 
aboral spike as a mere concomitant of the feeble develop- 
ment of the hybrid, a glance at the figures of normal 
Echinocardium larvae will be sufficient to refute this sugges- 
tion. We see there that the aboral skeleton and its formative 
