24 



MOLLUSCA. 



The development* of the embryo' is introduced by an unequal 

 segmentation. The segments arrange themselves in the form of a 

 blastosphere, on which the archenteron often arises by imagination, 

 while the rnesoderrn is developed from two cells which are early 



En 



a 



FIG. 502. Stages in the development of the larva of Teredo (after B. Hatschek). </, optic.-i 

 median'section of an embryo with two mesoderm cells (3Ix) and two entoderm cells (E-i) . 

 EC, ectoderm cells. 4, Ciliated embryo with mouth (O), stomach, intestine, and shell 

 gland (Sdr) ; S, shell, c, Later stage ; Sp, apical plate ; A, anal invagination. 



separated. The first 

 trace of the endoderm 

 also may have the form 

 of two cells (tig. 502). 

 The embryo, which is 

 partially ciliated and 

 often rotates within 

 the egg membranes, 

 soon acquires a ciliated 

 velum and shell gland. 

 The nervous system, 

 otocysts, and foot are 

 not differentiated till 



* Vide especially Loven, 

 " Bidrag till Kiinnedomen 

 <?, Larva of Teredo. O, mouth ; A, anus ; Pnc, prseoraJ om Utvecklingen af Mol- 

 ciliated ring ; Poir, postoral ciliated ring ; JV, pronephros ; J usca Acephala Lamelli- 

 Ot, otocyst ; Pg, pedal ganglion : Mz, mesoderm cells. branchiata." ' Stockholm 



1848. 



Flemming, *' Studien liber die Entwickelungsgeschichte der Najaden.'" 

 Sitzungsber. dcr K. Altud. da- W-ixscnsch. Vienna. 1S75. 



Carl Rabl, " Ueber die Entwickelungsgeschichte der Malermuschel." Jeua, 

 1876. 



B. Hatschek. " Ueber die Entwick-gesch. von Teredo." Ai-britr/t mix th-; 

 :/><>/. Institute, etc., Tom. III. Vienna. 1881. 



