46 PLATYHELMINTHES TURBELLARIA CHAP. 



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 auri- 

 culatum (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 the long tentacles which can be completely withdrawn, and in 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. 



The presence of a species (P. lemani) of the characteristically 

 marine genus Plagiostoma, in the Lake of Geneva, and in one or two 

 other Swiss lakes, at depths varying from 1 to 150 fathoms, is very 

 interesting, and is perhaps the only well-established case of the 

 survival of a once marine Ehabdocoelid under changed conditions. 

 Plagiostoma lemani is by far the biggest of the group to which it 

 belongs, being over half an inch in length. It is usually found in 

 fine mud, sometimes among Chara liispida, and has the general 

 appearance of an inactive white slug. "We are indebted to Forel 

 and Duplessis for the discovery of this species, and also of 

 Otomesostoma morgiense, a Mesostoma with an otolith, dredged in 

 10 to 50 fathoms in the Lake of Geneva, the Lake of Ziirich, and 

 found recently also by Zacharias in the Eiesengebirge. The genus 

 Bothrioplana, first found by Braun in the water-pipes of Dorpat, 

 has been carefully investigated by Vejdovsky, 1 who places it in 

 a special family, Bothrioplanidae, among the Alloeocoela. One 

 species has recently been found near Manchester. 



A comprehensive survey of the Rhabdocoelida shows that, 

 with the chief exception of the Proboscidae, the more lowly 

 organised forms, the Acoela and Alloeocoela, are marine, whereas 

 the fresh-water forms are in most cases the most highly organised 



1 Zeitschr.f. wiss. Zoologie, Bd. lx.,1895, p. 163. 



