TWO ABNORMAL PLUTEI OF ECHINUS. 
237 
have completed their development. I find that in these jars 
they develop very well until the so-called “ echinusHrudimeiit ,? 
has attained a large size, and then degeneration in many 
cases sets in. The ectoderm of the larval arms becomes 
retracted and gradually devoured by phagocytes, then the. 
Ci echinus-rudiment ” itself is attacked; the larva sinks to the 
bottom aud drags out an invalid existence for a week or two 
as a helpless cripple, consisting of merely the trunk and the 
larval alimentary canal. The cause of degeneration is some- 
how connected with the too luxuriant growth of the diatom 
in a confined space. 
On the other hand, the method which I employed in 1899 is 
very laborious, inasmuch as it requires constant removal of 
the bottom layer of the water in the culture jar, and its 
replacement by clean sea-water taken from a long distance 
off from the shore, and it is impossible to apply in an inland 
station. If, however, the larvas are brought on in the half- 
gallon jars till all the larval arms are fully formed, and then 
transferred to the eight-gallon jar with the pluuger, in which 
a culture of diatoms has been placed some time previously, 
then the happiest results are obtained. It is to be noted 
that if insufficient food be present in the large jar, the larval 
arms become fully developed and very long, but the “ echinus- 
rudiment ” remains in an iufantile condition, showing that 
the pathological absorption of the larval arms which occurs 
so frequently in the half-gallon jars is not due to want of 
food, but to toxic conditions connected with want of space. 
The abnormal larva shown in fig. 1 is normal in every 
respect save that of the echinus-rudiment. The eight arms 
pertaining to the fully .-developed Echinopluteus larva are all 
present aud long and equally developed. The larva is 
represented as seen from the dorsal surface, and one can see 
the long antero-lateral arms (a.Z.), and behind them the 
praeoral arms bending forward above the mouth (pr.o.). At 
the sides of the larva are seen the postero-dorsal arms (p.d.), 
whilst behind all tower up the aual or post-oral arms (p.o.). 
The whole course of the ciliated band can be seen. Below it 
