EXCRETORY ORGANS OF VERMES. 177 



In the Hirudinea these organs are preceded, in the embryonic 

 stage, by three pairs of looped canals, which are not connected with 

 those formed at a later stage ; they are found in the posterior half of 

 the ventral surface. In structure they are similar to, but simpler than 

 the permanent canals, and disappear after these are developed. This 

 most important fact shows that the looped canals of the Annulata 

 cannot be regarded as the direct homologues of the excretory organs 

 of the lower Vermes ; at the same time arises the question : Are the 

 looped canals of those Annulata, which show no signs of primitive 

 organs of this kind, comparable to the permanent looped canals of 

 the Hirudinea, or only to the primitive ones ? 



In then' more special characters there is great variety among the 

 Hirudinea, the canals in one division having no inner opening. 

 Instead of this they begin with a closed portion, of the form of a 

 loop, which consists of numerous canals, united with one another into 

 a labyrinth (Hirudo). From these looped organs a single canal is 

 given off, which opens by a vesicular enlargement on the surface of the 

 body (v. supra, Fig. 61, B). In others (Clepsine, Nephelis) the 

 labyrinthine portion is present, but it has an internal opening, which 

 projects into the lateral blood sinuses of the body. 



The division of the Limicoke is remarkable among the Scoleina, 

 because of the two different conditions of its looped canals. Usually 

 the canal is much-coiled, and the greater part of it is embedded 

 in a common mass of cells, and has much the same calibre throughout 

 its course. The canals always break through the dissepiment in front 

 of them by the end which carries the internal orifice, so that each pair 

 of looped canals is connected with two segments of the body. The 

 portion which leads to the exterior lies in one, the internal orifice in the 

 other. This form, which is distributed iu much the same way through 

 the greater number of the segments, is wanting in the tracts 

 occupied by the generative system. In place of the simple looped 

 canals we there find organs of greater complexity, and proportionately 

 much more developed ; in structure they resemble the others, but 

 they function as organs for the excretion of the sperm ; looped 

 canals are converted into seminal ducts. The same thing happens 

 in Branchiobdella. 



In the Lumbricida3 there are none of these physiological changes. 

 But the apparatus is greatly complicated by the marked development 

 of its separate divisions, as well as by the arrangement of the loops. 

 Each canal forms several ascending and descending loops, closely 

 connected with one another, and surrounded by a rich network of 

 blood-vessels. The different portions have different significations. 

 The innermost portion carries the funnel-like opening (Fig. 84, a), 

 and is followed by a part (b b b) which has transparent walls, and is 

 provided with cilia at various points. After forming several loops, 

 this portion passes, by a change in its walls, into another portion, 

 the lumen of which is widened (d), and surrounded by cells with 

 finely granular contents. This portion also has a looped course (d ), 

 and passes into a wider part, which is provided with muscular 



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