[Illustration: FIG. 38.—_Scolex polymorphus_ Rud. (larva of _Calliobothrium filicolle_ Zschokke), from the muscles of _Apogon_, a Mediterranean fish; also found in many Invertebrates (e.g. _Sepia_). A, Inverted scolex, with calcareous bodies; B, everted older larva. _br_, Brain; _exo_, terminal excretory aperture; _fc_, flame-cells; _for.sec_; secondary excretory pores; _hk_, hooks of the adult Cestode; _inrag_, pit at the bottom of which the head is developed; _msc_, anterior sucker; _nl_, lateral nerve; _sc_, suckers; _tl_, _tp_, lateral and main excretory vessels. (After Monticelli.)] OCCURRENCE OF CESTODES.—The distribution of Cestodes and their larvae is analogous to that of the digenetic Trematodes, although the absence of an alimentary canal limits the habitat of the mature worms to certain sites, such as the blood-vessels, the lymphatic and coelomic spaces, and the digestive system, where their body may be bathed by a nutritive fluid. Almost all groups of Vertebrates are attacked by Cestodes. Those of fishes, and particularly of Elasmobranchs, are distinguished by certain structural and developmental features; those of birds by {78}others; those of mammals, by a third set of characters. The young stages of the Cestodes of Sharks and Rays occur encysted in the body-cavity, or in the pyloric appendages, of Teleosteans, which probably swallow them along with those invertebrate animals upon which they prey. The larvae of the Cestodes of carnivorous mammals or piscivorous birds, live respectively in herbivores and fishes, but how the latter are infected we know in very few instances. Cestode larvae are known to occur in many Invertebrates, and occasionally are taken free swimming in the sea, presumably crossing from one host to the next. Ctenophores, Siphonophores, Copepods, Ostracods, Decapods, various Molluscs especially Cephalopods, Earthworms, and other Annelids, are the intermediate hosts of these larvae (see Fig. 38), the fate of which, however, has been determined in but few cases.