ENTOZOA. 



133 



there is a similar aperture which forms the 

 outlet of a vertically compressed sac situ- 

 ated between the chyle-receptacles (see Trans- 

 actions of the Zoological Society, plate 4, p. 381, 

 pi. 41, figs. 17, 18, d, g). In the Diplostomum 

 volvens Nordmann supposes the aperture in 

 question, h, to be the termination of a canal 

 continued from the oviduct. Besides this 

 canal the posterior appendage of the body is 

 occupied by a sac of a corresponding form 

 containing a milky fluid, i, t, and to which 

 the term of chyle-receptacle is given by Nord- 

 mann, as was previously done by Laurer to a 

 corresponding cavity in the Amphistoma coni- 

 cum. The nutritious contents of this canal 

 would seem to exude through the parietes of 

 the co3cal extremities of the intestines, as no 

 distinct aperture of communication is obvious. 

 Two vessels, k, k, are continued on each side 

 from the anterior and external part of the chyle 

 receptacle; they extend forwards to the anterior 

 third of the body, and are there brought in to com- 

 munication by a transverse vessel, /, /, which ex- 

 tends across the dorsal aspect of the body. From 

 the point of union of the transverse with the 

 external lateral vessels, a vessel is continued for- 

 ward on each side, appearing as the continuation 

 of the external lateral one. These vessels, m, m, 

 are reflected inward at the anterior angles of the 

 body, and unite in the middle line to form the 

 vessel, n, which may be regarded, according 

 to Nordmann, as representing the arterial 

 trunk, and which is continued to the posterior 

 extremity of the body, distributing branches on 

 each side throughout its whole length. Nord- 

 mann observed a circulation of fluid in the 

 vessels marked m, m, which was unaccom- 

 panied by any pulsation, and which may there- 

 fore be compared to the 

 cyclosis of the nutrient fluids 

 in the vessels of Polygas- 

 trica, Polypi, and other 

 Acrita, and is probably due 

 to the action of vibratile 

 cilia. 



In a few species of Pla- 

 naria the mouth is terminal 

 and anterior, as in the 

 Distomata ; these form the 

 subgemis Prostoma of 

 Professor Duges.* In the 

 greater number of these non- 

 parasitic Sterelmintha the 

 alimentarycanal commences 

 from a cavity situated at the 

 middle of the inferior sur- 

 face of the body. A pro- 

 boscis or suctorious tube (a, 

 jig. 82}, varying in length 

 according to the species, is 

 contained in this cavity, 

 from which it can be pro- 

 truded, and the mouth is 

 situated in the form of a 

 round pore at the extremity 

 of this proboscis. The ac- 

 tion of this tube is well dis- 



Fig. 82. 



Dendritic digestive 



cavity, Planaria 



lactea. 



* Duges, Annales des Sciences, 1828, p. 16. 



played when a hungry Planaria makes an attack 

 upon a Nais ; it then wraps its flat body around 

 its prey (see Jig. 76,) and applies to it the extre- 

 mity of its trumpet-shaped sucker ; the red- 

 blood of the little Anellide is seen to dis- 

 appear from the part in contact with the sucker; 

 and if the body of the Nais be broken in the 

 conflict, the Planaria directs the extremity of 

 the proboscis to the torn and bleeding surface. 

 After a meal of this kind the digestive canals 

 of the Planaria are displayed by the red colour 

 of their contents, like the corresponding parts 

 of the Liver-fluke when filled with bile, and 

 they greatly resemble the latter in structure ; 

 instead of two canals, however, three are con- 

 tinued from the base of the proboscis ; one of 

 these is central (6), and passes upwards to the 

 anterior extremity of the body, distributing its 

 wide coeca on either side ; the other two (c, c) 

 descend, almost parallel to one another, and 

 give off their coecal processes chiefly from the 

 outer margin, as in the Distoma. The Plarutrue 

 are, equally with the parasitic Trematoda, de- 

 void of an anus : and the remains of Poly- 

 gastric infusories swallowed by them have been 

 seen to be regurgitated by the proboscis. Mi- 

 nute nutrient vessels are continued from the 

 extremities of the intestinal cceca, and form a 

 very fine cutaneous network, which communi- 

 cate with a mesial and dorsal canal and two 

 lateral vessels, as in the Diplostomum. 



Some species of the Trematode Entozoa are 

 infested by parasitic Polt/gastrica which belong 

 to the Monads : Nordmann observed some 

 brown corpuscles by the sides of the alimen- 

 tary canal of a Diplostomum, which contained 

 minute particles in continual and lively motion. 

 On crushing the corpuscles between plates of 

 glass an immense concourse of the moving 

 atoms escaped : they were smaller than the 

 Monas atomos of Miiller, of an oval form, and 

 of a clear yellow colour; their movements were 

 very singular : they whirled rapidly round on 

 their axis, then darted forward in a straight 

 line, whirled round again, and again darted 

 forward. When we consider that the Diplos- 

 tomum itself does not exceed a quarter of a 

 line in length, and that the aqueous humour 

 of a single eye serves as the sphere of existence 

 to hundreds of individuals, what views does 

 the fact of the parasites of so minute an Ento- 

 zoon open of the boundless and inexhaustible 

 field of the animal creation ! 



Acanthocephala.The worms of this order, 

 although in external form, in the development 

 of the tegumentary and muscular system, and 

 above all in their direcious generation, they ap- 

 proach very closely the Nematoid Worms, yet 

 preserve the distinguishing character of the 

 Sterelminthoid class in the structure of the 

 digestive organs. In the Echinorhynchus 

 gigas the mouth is an extremely minute pore, 

 situated on a projectile armed proboscis, the 

 structure of which we have already described. 

 From its posterior part are continued two long 

 cylindrical canals (e, e, Jigs. 83, 84) which ad- 

 here closely to the muscular fibres by their outer 

 side, and project on the opposite side into the 

 triangular cavity (h, jig. 84) left between the 



