ECHINODERM LARVAE. 489 



associated in position with one of the primary tube-feet, 

 not lodged in any depression or cavity of the test, but 

 projecting freely from its surface (fig. 60). 



Spatangus jpuvpureus occurs not uncommonly in the 

 deeper waters in the neighbourhood of Port Erin, but I 

 have not recognised its larva. Mortensen (13) obtained 

 at Plymouth ripe eggs in the latter half of June, and 

 was able to rear a number of larvae until they were three 

 weeks old. As the breeding seasons of the various marine 

 Invertebrates appear to occur in the Irish Sea and the 

 English Channel at about the same times of the year 

 one would expect to find this larva in the plankton in 

 July. 



Figs. 61 to 64, PL IX, represent four stages in the 

 development of a Spatangoid echinopluteus which has 

 occurred in abundance in the plankton in the latter part 

 of February during the past four years, and again in 

 1914. It was abundant throughout March, 1914, and 

 numbers of fully developed specimens occurred in 

 the plankton about the middle of April. There 

 are in the Irish Sea only two known species 

 with which this larvae can be correlated, viz., Echino- 

 cardium flavescens and Brissopsis lyrifera, both inhabi- 

 tants of moderate depths and apparently somewhat locally 

 distributed. On comparing my figures with those of 

 Fuchs (2) of the early stages of the pluteus of Echino- 

 cardium cordatum, I find so much similarity in the 

 disposition of the skeletal structures of the two forms 

 that I think it probable that the former represent stages 

 in the development of Echinocardium flavescens. I have 

 not seen a stage later than that shown in fig. 64, and the 

 absence of any indication of the development of postero- 

 lateral arms shows that even this is an early stage of the 

 echinopluteus. In the case of either of the above-nnmed 



