252 On the Larval Forms of Spirorbis borealis. (March, 
the young of the Spirorbis which he studied leaves the egg cap- 
sule with its cephalic appendages much more developed than I 
have observed them to be in S. borealis. No branching appen- 
dages were seen in the youngest larve found free in the water in 
my specimens. 
The free larva seen from the ventral side is represented in Fig. 
10. It is easily detected in the water on account of its reddish 
color, although its size is not more than 1™ in length. The 
larva is more elongated and more vermiform than in previous 
conditions, and the middle body region is relatively much larger 
than formerly. It is no longer bounded posteriorly by the poste- 
rior edge of the collar. 
The head bears four! eye spots ; two larger, the original ocelli 
(oc), and two small, which are apical, dorsally and -medially 
placed. There is an apical tuft of cilia. The cilia of the belt, 
between the head and body, are borne on a ferrule-shaped struc- 
ture which separates the head from the middle body region, and 
which bears a small ring of cilia on its posterior, a larger on its 
anterior edge. The mouth opens medially just below this ferrule 
and above the base of the ventral collar. Its lips are richly cili- 
ated. The ventral collar covers about one-third the middle body 
region, which is not segmented and bears three pairs of single 
spines which are falciform at their free extremity. The middle 
region may be homologized with the anterior body region of 
such a worm as Prionospio. 
_ The posterior body region is of smaller diameter than the mid- 
dle and is obscurely segmented. It is ciliated on the ventral side, 
a prominent ring, “couronne ciliaire abdominale,” being formed 
on the segment nearest the middle division. The posterior ex- 
tremity of the larva is richly ciliated and bears several short stiff 
hairs. The whole body cavity, with the exception of that found 
in the posterior region, has a brick red color. When this larva 
_ is seen in profile it will be noticed that the external surface of the 
collar is ciliated over its whole extent, and that the prominent 
red projection on the ventral surface of the middle region is also 
1 Pagenstec echer says (of. cit, p. 492): “Um diese Zeit bilden sich aus einigen 
_ Zellen der äusseren Schicht des vordersten Lappens vier Augenpunkte, von denen 
_ die hinteren grösser sind, und auf der Mitte der Stirn wachst eine erst ungemein 
oye Soe gerade Borste hervor.” 
a A. Agassiz says: “ The ocular spots are mS zy to two.” The larvæ which 
i have studied have four and a median escribed by Pigihstecher 
