MUSEUM OF COMPARATIVE ZOOLOGY. 173 
Spio sp. 
Plate Il. 
The embryos of a species of Spio are among the most common larval worms 
found at Newport. They are very characteristic in form, and on that account 
are seldom confounded with the larve of other genera. 
The youngest Spio (Fig. 3) is telotrochal, and has a large, preoral lobe 
bearing an equatorial ring of cilia and embryonic spines, which arise from ear- 
like backward projections of the head. There are no paired cephalic append- 
ages, and no cephalic eye-spots, although scattered pigment marks the future 
position of the latter organs on the dorsal region of the head. The embryonic 
spines are about double the length of those on the body. Each embryonic 
spine, even when slightly magnified, is found to bear small lateral spurs at 
regular intervals along its length. When the larva is alarmed, the spines are 
raised, and project at all angles to their point of origin. 
The body is unsegmented, and, like that of other telotrochal Annelid larve, 
bears at its posterior end a ring of vibratile cilia, which arise from a thickly 
pigmented caudal segment. 
In a larva still older* (Fig. 1) than the last, several marked changes have 
occurred. One of the most important of these is a division of the body into 
somites, although no parapodia are yet visible along the lateral lines. When 
seen from the dorsal side, the ridge which bears the ring of cilia will be 
observed standing out more prominently from the body than in early condi- 
tions.. Along the anterior or upper part of this ridge there is a row of pigment 
spots. Slightly removed from the median line, and a little in advance of the 
ciliated ridge last mentioned, there are four eye-spots, called lateral ocelli. 
An additional pair of median eye-spots is placed near together on a slight 
backward extension of the head, behind the ciliated ridge. These, apparently, 
are wanting in a similar larva of Nerine,t of about the same age. 
The cephalic appendages (Fig. 2) are short and blunt, and have a length of 
about one third that of the body. They correspond to the dorsal cirri of the 
segment which forms the head. Small ventral cirri of the same segment are 
also found on the same side of the head as the mouth. The body consists of 
five segments and a terminal joint, which bears a well-marked circle of anal 
cilia. Each intermediate body segment is pigmented in the following pattern. 
When seen from the dorsal side, five narrow parallel bands of black pigment 
extend across the body in the interval between a line on the body opposite the 
extremity of the dorsal cephalic cirri and the anal circle of cilia. Each of 
these lines corresponds to a body segment, and, extending through about one 
* This larva is of about the same age as that figured and described by Leuckart 
and Pagenstecher in Miiller’s Archiv for 1858, Taf. XXIII. 
t See A. Agassiz, On the Young Stages of a Few Annelids, Ann. Lyc. Nat. Hist., 
Vol. VIII., 1866. 
