M. Sars on the Development of the Annelides, 185 



form of a membrane, similar to the so-called membrana nidulans 

 of Burdachj connects all the ova as it were by means of a tena- 

 cious mucus. This fringe is seen now and then to move slowly, 

 and curve in a worm-like form, drawing the e.^^ with it back- 

 wards and forwards. The cause of this motion remains a 

 mystery to me, if it be not owing to the action of the water on 

 the mucous substance of the fringe. The foetus itself, which gra- 

 dually acquires a bright gi-ayish green colour, was still without 

 motion in most of the ova ; only in a few a circle of extremely 

 minute, projecting and vibrating cilia was perceptible, which sur- 

 rounds horizontally the centre of the body of the foetus at an 

 equal distance from the two poles of the ovum. 



At last the foetus is arrived at maturity, and the mother now 

 carries on its back many thousands of young ones (fig. \% aaa), 

 which gradually come forth from the mucus surrounding the eggs, 

 leave their mother and swim freely about in the water, visible to 

 the naked eye as very minute greenish gray points (g'o^h of a 

 millimeter in size) endowed with a lively motion. 



The young, which have just left the shell (fig. 18, 19), are ex- 

 tremely unlike the mother both in form and in structure. They 

 are short, oval, cylindrical, unarticulated, and have, as above men- 

 tioned, horizontally round the centre of the body, a circle of tole- 

 rably long cilia [d d), in other respects however without any ex- 

 ternal organs. The portion of the body situated anteriorly to the 

 ciliary circle is somewhat narrower than the hinder one, and 

 bears two eyes {e e), and should therefore without doubt be con- 

 sidered as the head, the more so as the young one always swims 

 with this extremity in front. The eyes are at some distance from 

 the anterior free extremity (Z>), in the vicinity of the circle of cilia, 

 one on each side and a little towards the back; they are very 

 large m proportion to the body, black, and slightly elongated 

 diagonally, or almost kidney-shaped, with the convexity turned 

 anteriorly; not a trace of tentacula or antennae is observable 

 about the head. 



We just now called the side where the eyes approach nearest 

 together the dorsal side, while the opposite one, which moreover, 

 when the young is regarded from the anterior extremity, is some- 

 what more projecting (fig. 19 «), is proved to be the ventral side, 

 from the fact, that on it, close behind the circle of cilia, there is 

 an aperture (fig. 18 «), which I look upon to be the mouth. This 

 mouth-aperture is a diagonal fissure, whose lips are provided with 

 vibratory cilia, which are however much smaller than those of the 

 circle of cilia. There are also some very minute cilia at the most 

 anterior extremity of the head (fig. ISb). The intestine, as far as I 

 could observe from the slight transparency of the body, appears 



