24 



MOLLUSCA. 



The development* of the embryo is introduced by an unequal 

 segmentation. The segments arrange themselves in the form of a 

 blastosphere, in which the archenteron often arises by imagination, 

 while the mesoderm is developed from two cells which are early 



FIG. 502. Stages in the development of the larva of Teredo (after B. Hatschek). a, option! 

 median section of an embryo with two meaoderm cells (Ms) and two entoderm cells (En) 

 EC, ectoderm cells. b, Ciliated embryo with mouth (0), stomach, intestine, and shell 

 gl:md (&?); S, shell. c, Later stage; Sp, apical plate; A, anal invagination. 



Prio 



Jlfz 



rf 



d, Larva of Teredo. U, mouth ; A, anus ; Prw, prseoral 

 ciliated ring ; Poic, postoral ciliated ring ; N, pronephros ; 

 Ot, otocyst ; Pg, pedal ganglion; Mz, mesoderm cells. 



separated. The first 

 trace of the endoderm 

 also may have the form 

 of two cells (fig. 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 Lovn, 

 ill 



"Bidrag till Kannedomen 

 om Utvecklingen af Mol- 

 lusca Acephala Lamelli- 

 branchiata." Stockholm, 

 1848. 



Flemming. " Studien iiber die Entwickelungsgeschichte der Najaden." 

 Sitzungsber. dcr K. Akad. der Wissensch. Vienna, 1 875. 



Carl Rabl, " Deber die Entwickelungsgeschichte der Malermuschel." Jena, 

 1876. 



B. Hatschek. " Ueber die Entwick-gesch. yon Teredo." Arbeitcn aus Jem 

 zool. institute, etc., Tom. III. Vienna, 1881. 



