Some Rhabdocoels are parasitic. _Fecampia erythrocephala_, which occurs in the lacunar spaces and alimentary canal of young shore crabs (_Carcinus maenas_), is a white cylindrical animal ¼ inch long, with a red snout. After attaining maturity it works its way out of the crab and encysts under stones, forming a pyriform mass in shape like a "Prince Rupert's drop." Within this case the eggs develop, and the young probably emerge through the open narrow end of the hard white tube, but how they reach the crab is not known. _Graffilla muricicola_ is found in the kidney of _Murex brandaris_ and _M. trunculus_, at Naples and Trieste; _G. tethydicola_ in the foot of _Tethys_. _Anoplodium parasiticum_ occurs among the muscles which attach the cloaca of _Holothuria tubulosa_ to the body-wall; and _A. schneideri_ occurs in the sea-cucumber, _Stichopus variegatus_. These are truly parasitic forms, constituting a special sub-family. They have no rhabdites in the skin; the nervous system and sense-organs are only slightly developed; and the pharynx has undergone a notable reduction in relation to the simpler mode of obtaining nourishment. Other cases of association between certain Rhabdocoels (closely allied to, if not identical with, certain free-living species) and Lamellibranchs or Sea-urchins, are, however, of another kind. Thus on the gills or in the mantle cavity of species of _Mytilus_, _Cyprina_, _Tellina_, and upon the test of _Clypeaster_, such forms as _Enterostoma mytili_, _Acmostoma cyprinae_, and _Provortex tellinae_ have been found. But it is probable that these Turbellaria here obtain merely a temporary {46}shelter and possibly a supply of the food of the mussel or sea-urchin. The _Alloeocoela_ afford a well-established case of association. _Monotus fuscus_ (Fig. 19, D), an abundant, active, elongated animal, lives on our coasts in the upper part of the littoral zone among _Patella_, _Balanus_, and sometimes _Chiton_. When the tide is low, the _Monotus_, to obtain moisture and darkness, creeps between the mantle-folds of these animals, where it may readily be found. Upon the return of the tide it leaves its retreat and creeps or swims about freely. Other Alloeocoela collect in great numbers in tufts of red-seaweeds (Florideae). By placing such tufts in vessels, the sea-water, especially as darkness sets in, begins to swarm with _Cylindrostoma 4-oculatum_, species of _Enterostoma_ and _Plagiostoma_; _P. vittatum_, with three violet bands across the white body, being a particularly obvious form. _Vorticeros auriculatum_ (Fig. 19, C), another abundant species, is remarkable for the long tentacles which can be completely withdrawn, and in this condition it completely resembles a _Plagiostoma_.