RESPIEATOEY SYSTEM OF THE LEECH. 223 



nelidans, the organs provided for respiration are a series of membranous 

 pouches, communicating externally by narrow ducts, or spiracles, as they 

 have been termed, into which aerated water is freely admitted. These 

 sacculi, in the Leech, are about thirty-four in number, seventeen being 

 visible on each side of the body ; they are extremely vascular ; and in 

 connexion with every one of them there is a long glandular-looking 

 appendage, represented in fig. 110, m. 



(582.) According to the views of M. Duges, which, previous to the 

 appearance of Dr. Williams's interesting memoir, were received with 

 general assent, the two lateral vessels in the Leech are appropriated to 

 the supply of this respiratory system, and in them the blood moves in 

 a circle quite independent of that formed by the dorsal artery and 

 ventral vein, although they all communicate freely by means of cross 

 branches, those passing from the lateral vessels to the dorsal being called 

 by M. Duges* dorso -lateral, while those which join the lateral trunks 

 to the ventral canal are the later o- abdominal branches of that observer. 

 The movement of the blood in the lateral or respiratory system of vessels 

 is quite distinct from that which is accomplished in the dorso-ventral 

 or systemic trunks : sometimes it passes down one of these vessels from 

 the head towards the tail, and in an opposite direction on the other 

 side of the body ; but in a short time the movement of the currents will 

 be seen to become completely reversed, so that an undulatory motion, 

 rather than a complete circulation, is kept up. By this action of the 

 lateral canals the blood is made perpetually to pass and repass the 

 respiratory sacculi ; and, opposite to each of these, branches are given 

 off which form so many independent vascular circles, representing very 

 closely the minor or pulmonary circulation of higher animals. 



(583.) On examining attentively one of the " respiratory pouches," 

 according to the same authority, its membranous walls are seen to be 

 covered with very fine vascular ramifications (fig. 109, /), derived from 

 two sources : the latero-abdominal vessel (d) gives off a branch (e), 

 which is distributed upon the respiratory sacculus ; and there is another 

 very flexuous vascular loop (&), derived from the lateral vessel itself (a), 

 which terminates by ramifying upon the vesicle (/) in a similar manner. 

 The walls of the loop (6) are extremely thick and highly irritable; 

 but on tearing it across, the internal cavity or canal by which it is per- 

 forated is seen to be of comparatively small diameter ; so that we are not 

 surprised that, although such appendages to the respiratory sacs were 

 detected and well delineated by former anatomists f, their nature was 

 unknown, and they were supposed to be glandular bodies appropriated 

 to some undiscovered use. From the arrangement above described, 

 M. Duges was led to believe that small circular currents of blood exist, 



* Ann. des Sci. Nat. vol. xv. 



t Delle Chiaje, op. tit. Moquin-Tandon, Monographic sur la famille des Hiru- 

 dinees. 4to, Montpellier, 1827. 



