228 ANNELIDA. 



Leech, however, neither the vesicle, nor the tube into which it is pro- 

 longed, is ciliated, but, in lieu, they are highly contractile. The vesicle 

 is the respiratory sacculus of Duges ( 581). It does not, as Duges con- 

 tends, communicate externally by an orifice in the integument, but in- 

 ternally. The tube which supports this vesicle arises from that limb of 

 the segmental organ which contains the ova in their least-developed 

 condition. It is probable therefore that the chylaqueous fluid which this 

 tube is designed to discharge flows throughout the entire extent of the 

 organ, from the point of entrance to that of exit, before it escapes exter- 

 nally. In this course the fluid may afford the materials of nourish- 

 ment to the ova as they travel from one limb to another of the looped 

 organ. 



(595.) The organ itself may be described as a looped tube, the limbs of 

 which, returning upon themselves, and one invaginating the other, open 

 externally at the same point. In order to examine further these deli- 

 cate organs, they must be cut out, as closely to the abdominal surface as 

 possible, by means of a curved pair of scissors. They should then be 

 floated in water on a glass slide, covered by a thin slip, and placed 

 under the microscope. If they have been successfully dissected out, it 

 will be seen that one limb is broader and more transparent than the 

 other : in the former limb the ova are merely pellucid cells, consisting 

 exclusively of the germinal vesicle and germinal spot ; in the latter 

 limb the ova have acquired their full complement of vitellus. At this 

 stage they exhibit a flattened ovoid form. This fact affords a demon- 

 stration as to the course taken by the ova in their passage from the 

 ovary, properly so called, towards the exterior. The seat of the true 

 ovary, or that part which is endowed with the special power of ovo- 

 genesis, is marked by the plexus of vessels. In relation to this special 

 centre, all the rest of the organ may be characterized as the oviduct ; but 

 it is also a vitellarium. It follows therefore that, in the Leech tribe, the 

 ova at no time reach the cavity of the body : this may be one reason 

 why, in this family, the perigastric cavity is so contracted, and the con- 

 tained fluid so reduced in volume. 



(596.) ABBANCHIA TEKRICOLA. The species belonging to the second 

 division of those Annelidans which possess no external organs of respi- 

 ration are easily distinguishable from the suctorial worms by the dif- 

 ferent construction of their instruments of locomotion. They live in 

 general beneath the surface of the ground, either perforating the soil in 

 all directions, as the Earthworms (Lumbrici), or burying themselves in 

 the mud upon the sea-shore or of freshwater streams, where many of 

 them, called Naides (Nais, Linn.), live a semi-aquatic life. In con- 

 formity with such habits, their entire structure is adapted to a subterra- 

 nean existence, and their bodies are so organized as to enable them to 

 burrow with facility through the dense and unyielding materials wherein 

 they are usually found. Whoever has attentively watched the opera- 



