348 



INVEETEBEATA 



CHAP. 



divide into masses which, in the latest stages examined by Meisen- 

 heirner, are found beneath the pericardium and lying not far from 

 the lateral ectoderm on each side. The formation of the genital ducts 

 was not observed by him (Fig. 278). 



per.gl 



FIG. 278. Transverse section through a young Dreissensia polynwrpha in order to show 

 the origin of the genital organs. (After Meisenheimer. ) 



G, median mass of genital cells forming a thickening in the floor of the pericardium ; II, heart ; 

 int, intestine ; fc, kidney ; per.gl, pericardial gland ; per, pericardium ; r.p, reno-pericardial canal. 



Our account of the development of Dreissensia is now complete. 

 We must pause, however, and glance at what is known of the develop- 

 ment of other Pelecypoda before considering the development of the 

 highest Mollusca. 



OTHER PELECYPODA 



The development of no other Pelecypod has been worked out with 

 anything like the same completeness as Dreissensia. What we know 

 of other life-histories are mainly bits and scraps. From the accounts, 

 however, given by Horst (1882) of the development of Ostrea, by 

 Drew (1906) of that of Pecten, by Hatschek (1885) of that of Teredo, by 

 Sigerfoos (1895) of that of Pholas, and by Lb'ven (1848) of that of 

 Cardium, we can only conclude that the development of all these forms 

 is practically identical with that of Dreissensia. The figures given of 

 the veliger larvae are so similar that one would almost be driven to con- 

 clude that there is a veliger larva of definite type common to all marine 

 Pelecypoda, and that the differentiation of the various genera from one 

 another takes place during post-larval life. Indeed, the researches of 

 Stafford (1910) on the veliger larvae found in the lagoons of Prince 

 Edward Island, and off the New Brunswick coast, have gone far to 

 bear out this conclusion. Amongst other things he has shown that 

 the late veliger larva of Ostrea virginiana possesses a well-marked 

 foot which is used for locomotion in the early post-larval stages, 

 before the definitely fixed life of the adult is assumed (Fig. 279). 



