RINGED-WORMS. 213 
Respiration 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 DuGs) a blood-vessel in its iterior. 
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 Branpr 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 thana 
hundred such vesicles ; their openings are on the abdominal surface, 
according to Leo and Duas&s, whilst Mecken and Morren think 
that they are connected with a single series of apertures on the 
dorsal surface, which Writs formerly described and compared to 
the spiracles of Insects}. 
The ringed-worms, until within the last few years, were sup- 
posed, almost universally, to be bisexual. It was only in the 
Aphrodite 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?. Afterwards RATHKE also found 
a separation of the sexes in Amphitrite®, and QUATREFAGES observed 
the same in a large number of marine ringed-worms (tubicole and 
errantia)*. 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 Minne Epwarps, Ann. des Se. nat. sec. Série, Tom. x. 
pp: 193—221, Pl. ro, 11. (These figures are also transferred to the new edition of 
Cuvier, Régne Animal, Annelides, Pl. 1, &c.) 
1 De Anima Brutorum, Amstelodami, 1674, 8vo. pp. 34, 35, Tab. Iv. fig. 3. 
2 Misc. Zool. p. go. 
3 Beitrage zur vergl. Anat. u. Physiol. Danzig, 1842, s. 66—68. 
4 Minne Epwarps, Rapport sur une Série de Mémoires de M. A. DE QUATRE 
FAGES, Ann, des Sc. nat, 3itme Série I. p. 21. 
