176 BULLETIN OF THE 



visional setoe, yet wants the other cephalic aj^pendages of the larval Spio. In 

 other words, the appendages, which have been honiologized with the dorsal 

 cirri of a head segment, are never developed in this genus, while the spines 

 are formed only to disappear in subsequent growth of the worm as it matures. 

 The median antenna and other cephalic appendages are subsequent growths, 

 and may be looked upon as appendages of the originally single segment. 



Polytrochal Larva. 



Plate VI. 



A single polytrochal larva was taken about the end of the summer. The 

 body is elongated, wormlike, and bears on the sides of the head two flat circu- 

 lar ear-like appendages (" auricles "). Two small well-marked median cepha- 

 lic eye-spots are well developed. There are no tentacles, palpi, nor tentacular 

 cirri. The rudiments of two appendages resembling tentacles yet of a circular 

 shape point to a relationship between this larva* and some member of the 

 Spionidce. The body is composed of twenty-four segments. Tbe posterior 

 terminal body segment ends in two protuberances. Each of the body seg- 

 ments is pigmented on its posterior border. Along the dorsal region of the 

 larva there is also on the median line a similar although larger mass of pig- 

 ment. There are ten small pigment spots, ring-shaped and of black color, 

 which are i)laced at regular distances on one side of the body. Each lies on 

 a ciliated segment, and is situated in a median line on the ventral side of the 

 larva. 



The pigmental rings (j) ?•) on the ventral side of the body of the above- 

 described polytrochal larva seem to have a morphological meaning. At th 

 same time with our collecting of Sj)^ larvae we captured many yo.ung worms 

 (Figs. 2, 3, 4), which in outward appearance resemble very closely the older 

 stages in the growth of the same. These larvag, however, have one very in- 

 teresting difference, which allies them more intimately Avith the polytrochal 

 larva described above. As in many Annelid genera, Phyllodoce, Prionospio, 

 Telepsavus, and others, the fourth body segment is modified by the introduction 

 of diflfereut-shaped spines, or by pigmentation of the body walls, we naturally 

 expect in other genera to find this segment individualized as compared with its 

 neighbors. In a worm larva, somewhat resembling the polytrochal just de- 

 scribed, we find on the ventral side of the fourth body segment a pair of pig- 

 mented eye-spots (ocelli?), and that the fourth body segment bears a belt of 

 large cilia similar to those found on the head and caudal extremity. While 

 we recognize in these pigment spots the homologue of the ring-shaped pig- 

 mented regions in the median ventral line of the polytrochal larva, we can see 

 in this larva an intermediate larval form between the telotrochal and poly- 

 trochal conditions. The pigment spots are probably homologous with the 

 pigmented regions (jas) already described in Prionospio^ where, however, they 

 are forced more laterally, and the ciliated ring on the same segment is lost. 



e 



