MUSEUM OF COMPARATIVE ZOOLOGY. 173 



Spio sp. 



Plate II. 



The embryos of a species of Sjno 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 larvse of other genera. 



The youngest Spio (Fig. .3) is telotrochal, and has a large, prseoral lobe 

 bearing an equatorial ring of cilia and embryonic spuies, 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 larvae, 

 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 jDart 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 pan- 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 Ncrine,-\ 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 Ime 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 



* Tliis larva is of nbont tlic snmc nso as that fignrpil nnd described by liCUckart 

 and Pageiisteclier in Miiller's Arcbiv for 1858, Taf. XXII I. 



t Sec A. Airnssiz, On the Young Stages of a Few Annelids, Ann. Lye. Nat. Hist., 

 Vol. VIII., 1^G6. 



