* 
254 On the Larval Forms of Spirorbis borealis. — (March, 
peared, the most important is a pair of the ocelli, the first to origi- 
nate and fora long time the most prominent eye spots found 
on the ventral cephalic prominences. The apical ciliated tuft 
of former stages has also disappeared. The apical eye, spots 
still remain. 
Of the appendages to the head we notice on the right hand 
side instead of the club-shaped tentacle which formerly existed 
there, that the place is now occupied by an elongated body with 
beginnings of side branches, a structure which later forms a 
branchia. On the left hand side, near'the operculum, are small 
projections which later develop into the left hand branchia, while 
medially appear two prominences, one upon the other. In the 
development of the larva of Spirorbis it looks as if we had, as I 
have already shown, in a larva which is provisionally identified 
as the immature Prionospio, and as Salensky has found in Pileo- 
laria, temporary cephalic tentacles which later give place to the 
permanent branchiz of the head. In the right hand branchia, 
now of considerable size, we formerly had a small tentacle which, 
although it never reaches the great size of the temporary tenta- 
cle of Prionospio, is so closely similar both in size and general 
appearance to the temporary tentacle of Pileolaria, as described 
by Salensky, that it is in Spirorbis placed in the same category.’ 
The passage of the free larva of Spirorbis into the form with a 
case is a most interesting process, and one which is by no means 
simply in the changes involved. It can easily be observed in 
early conditions and the interior even of the larva studied, since 
the external case, when first formed, is almost wholly transparent. 
At this age the larva becomes attached to the walls of the ves- 
sel in which it is confined preparatory to the secretion of a shell. 
In many specimens, however, the following condition, which 
although probably abnormal, was most advantageous to a study 
_ of the secretion of the tube, was observed. The free larva often , 
1 Bull. Mus. Comp. Zodl., Vol. x1, No. 9. 
?I do not consider that the worm represented in fig. 57 of A. Agassiz’s paper (af. 
cit.) is, when compared with fig. 56, an instance of retrograde development. Fig. 
ete 57 bears a strong likeness to d/aurina prolifera commonly looked upon not as an 
>  amnelid but as a turbellarian. If the worm is an Alaurina I cannot regard the larva 
~ :Tepresented in fig. 56 as its young. The adult of fig. 56 is unknown, and it is ex- 
_ tremely doubtful that it ever loses its cephalic spines and appendages and passes into 
a fig. 57. It may or may not resemble beens sox in a subsequent modification of the 
os tentacles into branchiz. 
