174 



COMPARATIVE ANATOMY. 



appear to be excretory products. In the Trematoda these con- 

 cretions are collected into the large trunks, which contract and 

 drive them into the terminal vesicle, whence they are evacuated by 

 the porus excretorius. 



An anastomosis of the finest ramifications of the canals may be 

 often made out in the Cestoda, as well as in the Trematoda (Distoma 

 dimorphum), and may even affect the larger vessels, which either 

 become connected in a simple manner (into a ring in Distoma 

 rhachiasum, and into regularly succeeding transverse canals in many 

 Cestoda) ; or become converted into a rich rneshwork, in which the 

 chief trunks disappear. 



The excretory organs in the Nematkelminthes, which again may 

 be derived from a caecal tube, are simpler in character. They form 

 tubes or canals, which pass along the body embedded in the lateral 

 arete (Fig. 61, A r). Near the fore-gut the canals of either side bend 

 towards one another, and unite into a common portion of varying 

 length, which opens to the exterior by a pore in the ventral line. 

 Sometimes these canals are coiled, and they vary greatly in the 

 way in which they are connected with the pore. In the Gordiacea 



this apparatus appears to be rudimen- 

 tary, for in Mermis it is simply repre- 

 sented by a row of cells, and Gordius, 

 in which there are no lateral tracts, has 

 no distinct organ of this kind. 



It is doubtful whether the organs 

 found in the anterior region of the body 

 in the Acanthocephali, which are known 

 as " lemnisci/' belong to the excretory 

 system or not. They form two longish 

 lamellae, without a lumen, which are 

 processes of the body-wall, and like it 

 are provided with branches of canals, 

 between which dark granular masses 

 occur. 



§ 143. 



When a ccelom is formed, the cha- 

 racters of the excretory organs are so 

 far altered that the canals communi- 

 cate with it by internal ciliated orifices. 

 This new condition must be regarded 

 as nothing more than a modification of 

 the blindly-ending canal system, as it 

 has been already described in the Pla- 

 tyhelminthes. Internal openings have 

 been observed in the larvae of Trema- 

 toda. They characterise also the ex- 

 cretory canal system of the Rotatoria, which is disposed in just the 

 same way as in the Trematoda. The canal system, which lies in the 



a 



Fig. 81. Organisation of 

 Brachionus. a Ciliated cephalic 

 disc, s Siphon, m Masticatory 

 organs, e Glandular layer in the 

 stomach. o Ovary. u Uterus, 

 containing an egg. o' Eggs, 

 attached to the root of the tail. 

 c Excretory canals, v Contractile 

 terminal vesicle. 



