EXTOZOA. 85 



outlet to the digestive system, and the capillary vessels, extending 

 from the chyle-receptacles to that pore, as a ramified form of intes- 

 tine, fulfilling at the same time the office of lacteals, lymphatics, 

 arteries, and veins ; but an excretory function is doubtless the chief 

 one of this ramified system of vessels. 



In 3Ionostoma J7iutabile the two digestive canals bend towards, 

 and anastomose with, each other at the hinder end of the body. In 

 Aspidogaster the digestive canal is continued backwards, without 

 dividing, and ends by a cul-de-sac. A similar simple blind tube is 

 continued from the mouth of Gasterostoma, which, as that name 

 implies, is situated at the middle of the abdomen. In Distoma chi- 

 lostoma and other species from the abdomen of Neuropterous insects, 

 two short blind tubes diverge right and left from the gullet. The 

 ramified type of the alimentary canal, exhibited by Dist. hepaticuniy 

 is repeated in Octobothrium palmatuni, O. Merlangi, Poly stoma 

 appendiculatum, Tristoma elongatum, and in the genus Diporpa*, 

 which, in the state of conjugation f, represents that apparently most 

 extraordinary form of the present order, called by Nordmann, its 

 discoverer, Diplozoon paradoxum. In this Trematode, as well as 

 in some others, e. g. Distoma echinatum, Aspidogaster conchicola, 

 certain parts of the vessels show a ciliated inner surface, or special 

 ciliated processes extending therefrom, which actively vibrate, and 

 may relate to a respiratory process. 



The posterior contractile sac, already referred to in the Distoma 

 clavatum, is present under certain modifications in many Trematoda. 

 It is simple in Monostoma Faba, Distoma cirrigerum and Gastero- 

 stoma Jimbriatum ; is bifurcate in Distoma clavigerum, D. tereticolle, 

 D. variegattim, and in many Monostotnata, in which the two blind ends 

 of the sac reach almost to the head. If I have been right in regarding 

 the so-called "vascular system" of Dist. hepaticum as homologous 

 with this sac and its prolongations, we have, then, in that species, a 

 third (ramified) form of the excretory organ. An intermediate mo- 

 dification is presented in the Amphistoma cotiicum. The contents 

 of the excretory sac consist usually of a colourless fluid containing 

 many granules and vesicles : but sometimes it is fiUed by clear cal- 

 careous corpuscles like those found in the parenchyme of the cestoid 

 worms and their cystic larvae. These substances are excreted by the 

 terminal pore. In the above-cited lower organised Sterelmintha 

 the lime-corpuscles remain aggregated beneath the skin. 



The genus Planaria of Miiller is now known to include many 

 generic types of fresh-water vermiform animals, not internal parasites, 

 yet closely allied, by their organisation, to the order Trematoda. 



* LXXITL p. 316, pL 8. f. c. 200. f LXXFV. p, 63. 



G 3 



