166 Dr. E. Koischelt on the 



Since tliis larval organ lias not only l)ecn proved to exist in 

 the free-swimming Trochophora of Teredo (Hatschek), but 

 was also described for the very degenerate Trocliophora of 

 Cydas (Zieglcr), it is hardly open to doubt that it is present 

 in 7)yT?>.sT?Kt also ; the more so since the agreement of the 

 larvae of Dreissena with those of the marine Lamellihranchs 

 is in other respects complete — a fact to which I would call 

 attention once more. This ai)pcars in a peculiarly striking 

 fashion on instituting a com])arison with the figures which 

 l.ovdn {loc. cit.) gives of various larvaj (belonging to Gar- 

 cUum, Modiolaria, 3fo?iiacuio, and other genera which are not 

 further s])ccified). I'he larva in its youngest stages provided 

 Avith a bivalve shell exhibits a very remarkable agreement 

 with that of the Euroj)ean oyster, as figured by Huxley *, 

 ]\lobiust5 and llorst {loc. cit.). The shape of the shell, too. 

 at first resembles that of the shell of the young oyster-larva. 

 It is almost circular, though it aj)pears flattened on the dorsal 

 surface, owing to two straight lines which there come into 

 contact (fig. 1). It subsequently becomes more circular ; with 

 further growth the shell becomes arched ; this takes place 

 especially in the region of the cardo, and leads to the forma- 

 tion of the umbo. The shell is now not unlike that of a 

 Gardium. It still consists solely of a membrane of conchiolin ; 

 the secretion of the calcareous matter does not take place 

 until later. 



The larvaj rove about for some eight days, and during the 

 greater ])ortiou of this period are found chiefly on the upper 

 surface of the water. In Lake Tegel multitudes of pelagic 

 alga' W'Crc collected at the same time, and these doubtless 

 ])rovide the larvae Avith suflicient nourishment. Wiicn they 

 are still thoroughly ca])able of active progression by aid of 

 the velum they sink from the suriace to the bottom of the 

 water. This ma}' either be explained as resulting from 

 changes which now take ])lace in their organization or may 

 be traceable to the fact that food is perhaps easier of acqui- 

 sition in the depths. 



While the larva was still enjoying a pelagic existence 

 certain changes set in, of which the most important is the 

 formation of the foot. The latter appears as a protuberance 

 between the mouth and anus, and quickly assumes a conical 

 shape. It then grows in length, and is forced to become bent 



* II. Iluxloy, " Oysters and the Oyster-Question,'' Englisli Illustrated 

 Miif^azine, 188.'}. 



Co])ied ill Jackson's psipor " IMiylogony of the relocvjioda," Mem. Bost. 

 Sec. Nat. Hist. vol. iv. no. viii., 18i)0. 



t K. iMiibiiu^, ' I'ic Anstor und die Auslernwirlhschal't,' Berlin, 1877. 



