RINGED-WORMS. 213 



Eespiration is effected by the skin, or by external gills of very 

 different form, or by vesicles on the sides of the body. In the Leech 

 there are found about seventeen such vesicles on each side, which 

 open on the abdominal surface. The openings are extremely minute, 

 and between two of them on each side there are four rings or seg- 

 ments of the body without such openings. A white convoluted 

 structure is connected with these vesicles by means of a thin pedicle, 

 and contains (according to DUGES) a blood-vessel in its interior. 

 That these vesicles secrete mucus, is no proof that they are not 

 respiratory organs ; some writers think that it is their sole function 

 to supply that secretion; and BRANDT believes that respiration in 

 the Leech is effected by the skin. At all events, though these 

 vesicles receive and return blood-vessels, they have not a perfectly 

 separate circulation of blood in them, and the respiratory organs 

 would seem to receive in this case, as in that of Reptiles, a portion 

 only of the venous blood. In the Earth-worm there are more than a 

 hundred such vesicles ; their openings are on the abdominal surface, 

 according to LEO and DUGES, whilst MECKEL and MORREN think 

 that they are connected with a single series of apertures on the 

 dorsal surface, which WILLIS formerly described and compared to 

 the spiracles of Insects 1 . 



The ringed-worms, until within the last few years, were sup- 

 posed, almost universally, to be bisexual. It was only in the 

 AphroditcB that a separation of the sexes was, with some hesitation, 

 accepted, when PALLAS had shewn that certain individuals were full 

 of eggs at the same time that in others the cavity of the abdomen 

 contained a tenacious milky fluid 2 . Afterwards EATHKE also found 

 a separation of the sexes in Amphitrite*, and QUATREFAGES observed 

 the same in a large number of marine ringed-worms (tukicolce and 

 errantia) 4 . The observations of STEENSTRUP on Lepidonote, Phyllo- 

 doce, Nereis , Nephthys, Terebella, and Serpula are to the same effect: 

 in the last genus the sexual distinction may be recognised by the 



consulted, and especially MILNE EDWARDS, Ann. des Sc. not. sec. Sdrie, Tom. x. 

 pp. 193 221, PI. 10, ir. (These figures are also transferred to the new edition of 

 CUVIER, Reyne Animal, Annelides, PI. i, &c.) 



1 De Anima Brutorum, Amstelodami, 1674, 8vo. pp. 34, 35, Tab. IV. fig. 3. 



3 Misc. Zool. p. 90. 



3 Bdtrdge zur vergl. Anat. u. Physiol. Danzig, 1842, s. 66 68. 



4 MILNE EDWARDS, Rapport sur une Serie de Memoircs de M. A. DE QUATRE- 

 FAGES, Ann. des Sc. nat. 3ibme Sdrie I. p. 21. 



