Fig. 124.—Allocreadium isoporum, Looss. Excretory apparatus. Of the other organs, the oral sucker, pharynx, genital pore, ventral sucker, ovary and testes are shown; the cylindrical excretory bladder is in the posterior end. 38/1. (After Looss.)
The final products of assimilation dissolved in the fluids of the body are distributed throughout the parenchyma and are thence expelled by a definite tubular system (excretory apparatus, proto-nephridia, formerly also termed the water-vascular system). This system, which is distributed throughout the entire body (fig. 124), is symmetrically developed, and, in the ectoparasitic Trematodes, it opens, right and left, at the anterior end on the dorsal surface; in all other flukes, however, it opens singly into the excretory pore (foramen caudale) at the centre of the posterior border; in those cases, however, where a sucker is present at the posterior end, as in the Amphistomata, the excretory pore is situated on the dorsal surface close in front of the sucker.
The EXCRETORY SYSTEM[258] consists of several parts: (1) of the more or less numerous terminal “flame” cells or funnel cells (figs. 124, 125); (2) of the capillaries ending in them; (3) of larger vessels receiving the capillaries; and (4) of the excretory bladder. Terminal cells and capillaries may be compared to unicellular glands with long excretory ducts; the cellular body (fig. 125) is comparatively large, stretched longitudinally, more rarely transversely, and provided with numerous processes, that are lost in the parenchyma; within is a conical cavity (analogous to the secretory cavity of unicellular glands) which is continued directly into the structureless capillary; at its blind end is a bunch of cilia projecting into the cavity, and which, during life, shows a flickering motion (ciliary flame). The nucleus is situated in the protoplasm of the terminal cell at its blind end.
Fig. 125.—Terminal flame cell of the excretory system. n., nucleus of cell; c., bundle of cilia forming the “flame”; p., processes of cell extending into parenchyma; d., excretory capillary. (Stephens.)
The entire apparatus thus begins blindly—i.e., within the terminal cells, to which must be ascribed the capacity of taking up from the fluid that permeates the parenchyma the products which are first collected into their own cavities and thence excreted by means of the capillaries and vessels.
The vessels possess definite walls, consisting of a membrane and a nucleated protoplasmic layer. They unite at many points on either side, and again pass into other canals (COLLECTING TUBES), which finally, travelling towards the posterior end, discharge into the excretory bladder (fig. 124).