24-8 Mr. A. Agassiz on the Young Stages 



allied, if not identical, genus Spio. We find in both the large 

 brushes of stiff, serrated, temporary bristles observed by Clapa- 

 rede in his young LeucodorcE, which, at the least disturbance, 

 they spread fan-like in every direction, roll themselves up into 

 a ball like a hedgehog, and become quite motionless, but soon 

 start off again on their rapid gyrations, performed by means of 

 an exceedingly powerful circle of vibratile cilia surrounding the 

 head. The body at this early stage consists of seven distinct 

 rings, and faint indications of a couple more in front of the 

 anal ring, also surrounded by strong vibratile cilia. The dorsal 

 cirri are slight swellings, and at the base of each we find two 

 brushes of permanent bristles — the one composed of serrated, 

 file-like, rough bristles (fig. 42 ^), similar to those observed by 

 Claparede in an unknown Annelid larva*, the other of smooth 

 setae, like those of the adult, placed immediately at the base of 

 the rudimentary dorsal cirrus. The oesophagus and stomach 

 are separated by a slight constriction. The younger stages 

 (fig. 39) have no pigment-cells, and are moderately transparent; 

 there are six eyes, the two larger ones, placed nearer the sides 

 of the body, soon disappearing. 



In subsequent stages the body lengthens and becomes more 

 pointed; pigment-spots appear near the head, extending to- 

 wards the anal ring; they resemble those of Pohjdora, pass 

 through the same stages, and, before they disappear, have lost 

 their beautiful star-shaped form, making a double row of more 

 or less rectangular spots, as in fig. 40. The changes have 

 been principally in proportions ; the tentacles have slightly de- 

 veloped, the large anterior brush of serrated bristles losing its 

 prominence. The dorsal cirri, as well as the tentacles, now de- 

 velope rapidly, the powerful circle of vibratile cilia round the 

 head having nearly disappeared (fig. 41). We find in some of 

 the rings of the anterior part of the body the first appearance 

 of the clusters of stiff hook-shaped bristles, like those of fig. 44, 

 found with the lower brush of smooth bristles in the adult; 

 the eyes are four in number, quite small ; the pigment-spots 

 have disappeared, as well as the anterior brush of temporary 

 bristles. The little worm now enters a stage when it rapidly 

 assumes the appearance of the adult. The number of rings 

 increases rapidly; the dorsal cirri, as well as the tentacles, 

 lengthen materially; the stomach proper becomes much nar- 

 rower, and towards the anal extremity a distinct intestine 

 (fig. 42) has been formed. In a stage preceding (fig. 40) it 

 has many characteristics of the larva figured by Busch, on 

 plate 8. figs. 1 & 2 of his ' Beobachtungen.-' The serrated bristles 



* Beobachtungen, pi. 6. fig. 6. 



