40 HOWARD AYERS. 
To M. Joubin, in charge of the station, I wish to tender my 
thanks for many kindnesses shown me during my sojourn at 
Banyuls. 
At the beginning of the last decade Lovén (1) discovered 
and described certain club-shaped organs occurring among the 
well-defined group of Echinoderms, the Regularia or Echini, 
save the single genus Cidaris. None of the other groups of 
the Echinodermata have as yet been shown to be provided with 
these organs. The Swedish naturalist applied the very appro- 
priate name of Spheridia to these organs, and considered them 
to be sense organs—very probably for the perception of varia- 
tions in the chemical conditions of the surrounding medium. 
Lovén has figured and described the various forms of this sense 
organ from a majority of the genera of the Echinide, but does 
not enter into details of structure, except in the case of the cal- 
careous globule, and here his interpretations are wrong in many 
points, as I hope to show further on. In the present paper I 
shall give an account of the structure of the sphzeridia in the 
more important details, and wish to call special attention to the 
nervous structures of these organs. The observations at the 
Mus. Comp. Zool. were made chiefly on Strongylocentrotus 
droebachiensis, O. F. Miill., so common on the New Eng- 
land coast, while at the Laboratoire Arago the spheridia of the 
species named below were particularly studied: Echinus 
melo, Lam., E. esculentus, Linn., E. microtuberculatus, 
Blainv., Strongylocentrotus lividus, Brdt., S. droe- 
bachiensis, O. F. Miull., Spherechinus granularis, A. 
Ag., Spatangus purpurens, O. Fr. Mull. 
The number and position of the spheridia on the test is 
quite constant (except for the very young animal) for the 
species, and might be used, were it necessary, as a specific 
character. They are always confined to the ambulacral 
zones of the actinal surface of the Urchin, and are usually con- 
centrated about the peristome. When they are numerous, as 
in the Regularia,! they may extend in radiating lines to some 
1 Arbacia forms an exception to this general rule, since it has but five 
spheridia, one to each ambulacral zone, placed in depressions of the peri- 
