LAMELLIBRANCHIATA. 



343 



internal part, having the form of a ciliated canal, and of a superficial part which 

 opens externally on the posterior and ventral side of the cephalic region (Cyclas, 

 Teredo). 



The development of the fresh-water forms (Cyclas, Unio, Anodonta), in which 

 the eggs and embryos are contained in well protected brood-pojjches, may he 



En 



Mz 



FIG. 268. Stages in the development of the larva of Teredo (after Hatschek). EG ectoderm ; 

 En endoderm ; Ms mesoderm. a, median optical section of an embryo with two mesoderm 

 cells Ms and two endoderm cells En. b, ciliated embryo with mouth 0, stomach, intestine, 

 shell-gland Sdr and shell S. c, later stage with anal imagination A, apical plate Sp, and 

 more extensive shell S. d, Trochosphere larva of Teredo. mouth ; A anus ; Prw preoral 

 ciliated ring or velum ; Pow postoral ciliated ring ; N larval kidney or pronephros (head- 

 kidney) ; Ot otocyst ; Pg pedal ganglion ; Mz mesoderm. 



called direct. The marine Lamellihranchs on the other hand are set free at an 

 early stage and swim about for a long time as larvae with large umbrella-like 

 vela, from which the labial palps are developed. 



The Unionidae have a somewhat complicated development.* The eggs are 



* Schierholz, C. "Ueber die Entw. d. Unioniden," Denkschr. k. Akad. Wiss. 

 zu Wien, Math.-Naturw. Bd. 45, 1889. 



