274 ANNELIDA. 



each dorsal foot ; the outgoing limbs, considerably longer and more 

 tubular than the former, open externally to the median side of the root 

 of each ventral foot. The cilia with which this horse-shoe tube is 

 lined are highly vigorous, and capable of supporting a powerful current, 

 which arises externally and terminates externally ; but, as Dr. Williams 

 asserts, the ova in the female and the sperm-cells in the male escape, 

 although in some undetermined mode and by some undemonstrated 

 passage, from this organ into the complexly areolated tissue which fills 

 the chamber of the pedal appendages, which is a development of the 

 segmental organ, and in size and vaseularity is proportionate to the 

 stage at which the contained germinal elements have arrived. 



The following observations of M. Sars*, relative to the embryogenesis 

 of these worms, are extremely interesting and important : 



(705.) In Polynoe cirrata the months of February and March are the 

 period of propagation, when the body assumes a pale rose colour, arising 

 from a numberless quantity of eggs, which fill the abdominal cavity, with 

 the exception of about the first anterior fourth, and appear everywhere 

 through the skin. When the animal is opened, the eggs appear to hang 

 together in masses by means of a connecting tenacious mucus. In 

 other individuals the eggs occur on the top of the back of the mother, 

 beneath the dorsal scales, in immense numbers, surrounded by a tena- 

 cious mucus. The heaps of eggs cover the whole hinder half of the 

 back, but, more anteriorly, only the sides above the bases of the feet. It 

 would seem that the eggs pass out through a very small aperture just 

 above the feet, as Rathke found in the case of Nereis pulsatoria. Here, 

 protected beneath the dorsal plates, the eggs remain until the young 

 escape. In the meantime the yelk undergoes the usual process of mul- 

 berry-fission, until it becomes finely granular. The ova become slightly 

 oval ; and the foetus (into which the entire yelk is converted, without any 

 part whatever separating) is smooth, greyish white, and more or less 

 narrowly enclosed in chorion. A peculiar kind of motion was now ob- 

 servable on placing the separated ova under the microscope, the ova 

 turning round and round: this was effected by a very short fringe, 

 which is seen now and then to move slowly and curve in a worm-like 

 form, drawing the egg with it backwards and forwards. The foetus 

 itself, which gradually acquires a white greyish-green colour, is still 

 without motion in most of the ova : only, in a few a circle of extremely 

 minute projecting and vibrating cilia was perceptible, which surrounds 

 horizontally the centre of the body of the foetus, at an equal distance 

 from the two poles of the ovum. At last the foetus arrives at maturity, 

 and the mother now carries on her back many thousands of young ones, 

 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-grey points (^th of a millimetre in size) 

 * Sars, Wiegm. Archiv, 1845, part 1. 



