It is a mistake to assert that the young _Glochidium_ can swim. When they finally quit the mother, they sink to the bottom, and there remain resting on their dorsal side, with the valves gaping upwards and the so-called byssus streaming up into the water above them. There they remain, until a convenient ‘host’ comes within reach, and if no ‘host’ comes within a certain time, they perish. They are evidently peculiarly sensitive to the presence of fish, but whether they perceive them by smell or some other sense is unknown. “The tail of a recently killed stickleback thrust into a watch-glass containing _Glochidium_ throws them all into the wildest agitation for a few seconds; the valves are violently closed and again opened with astonishing rapidity for 15–25 seconds, and then the animals appear exhausted and lie placid with widely gaping shells--unless they chance to have closed upon any object in the water (_e.g._ another _Glochidium_), in which case the valves remain firmly closed.” In about four weeks after the _Glochidium_ has quitted its host, and the permanent shell has made its appearance _within_ the two valves of the _Glochidium_, the projecting teeth of the latter press upon the ventral edge of the permanent shell, at a point about half way in its lengthward measurement, retarding the growth of the shell at that particular point, and indenting its otherwise uninterrupted curve with an irregular notch or dent. As growth proceeds, this dent becomes less and less perceptible on the ventral margin of the shell itself, but its effects may be detected, in well-preserved specimens, by the wavy turn in the lines of growth, especially near the umbones of the young shell. Mr. Latter found that all species of fish with which he experimented had a strong dislike to _Glochidium_ as an article of food. Sometimes a fish would taste it “just to try,” but invariably spit it out again in a very decided manner. The cause of unpleasantness seemed not to be the irritation produced in the mouth of the fish by the attempt of the _Glochidium_ to attach itself, but was more probably due to what the fish considered a nasty taste or odour in the object of his attentions. * * * * * The following works will be found useful for further study of this portion of the subject:-- =F. M. Balfour=, Comparative Embryology, vol. i. pp. 186–241. =F. Blochmann=, Ueber die Entwickelung von Neritina fluviatilis Müll.: Zeit. wiss. Zool. xxxvi. (1881), pp. 125–174. =L. Boutan=, Recherches sur l’anatomie et le développement de la Fissurelle: Arch. Zool. exp. gén. (2) iii. suppl. (1885), 173 pp. =W. K. Brooks=, The development of the Squid (Loligo Pealii Les.): Anniv. Mem. Bost. Soc. Nat. Hist. 1880. „ „ The development of the oyster: Studies Biol. Lab. Johns Hopk. Univ. i. (1880), 80 pp. =R. von Erlanger=, Zur Entwickelung von Paludina vivipara: Morph. Jahrb. xvii. (1891), pp. 337–379, 636–680.