48 
The Post-larval Stage of Arenicola marina. 
By 
Wm. Blaxland Benham, D.Se.Lond., 
Aldrichian Demonstrator of Anatomy in the University of Oxford. 
With Plate I. 
By “ post-larval stage” I wish to indicate that stage in the deve- 
lopmental history of Arenicola at which the full adult number of somites 
has appeared, and the body is already distinguishable into (a) an 
anterior chetigerous region, and (b) a posterior acheetous region or 
tail, but in which the gills are not yet completely formed or have 
not yet even made their appearance. 
Such a stage was sent to me early in March, 1893, by Mr. Gar- 
stang, with a suggestion, which turned out to be perfectly well 
founded, that the worms were the young of Arenicola. 
My hearty thanks are herewith accorded to him for his kindness 
in giving me the opportunity of studying them. 
Two of these small worms were collected, one on February 22nd, 
the other on the 23rd of the same month, and he sent them to me 
preserved, One was stained and mounted entire, the other was cut 
into sections, partly longitudinal, partly transverse. An unfortunate 
accident to the longitudinal sections deprived me of investigating 
the anterior end of the worm as thoroughly as I could have wished, 
and though Mr. Garstang has been constantly on the look-out for 
more specimens, success has not crowned his efforts. 
Mr. Garstang gives me the following information as to these larve : 
—‘‘ Kach was inhabiting a perfectly colourless and transparent gelatin- 
ous tube, obviously secreted by itself. The body of the Arenicola larva 
was very flexible when alive, enabling it to wriggle actively in an 
eel-like manner in the water—generally near the surface—when placed 
in a tall clear glass jar.” 
