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 invagination, 

 while the mesoderm is developed from two cells which are early 



EC 



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

 median section of an embryo with two mesoderm cells (M*) and two entoderm cells (En) ; 

 EC, ectoderm cells. b, 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 (fig. 502). 

 The embryo, which is 

 partially ciliated and 

 often rotates within 

 Pou the egg membranes, 

 soon acquires a ciliated 

 velum and shell gland. 

 The nervous system, 

 otocysts, and foot are 

 not differentiated till 



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

 ciliated ring; Pow, postoral ciliated ring; N, pronephros; 

 Of, otocyst ; Pff, pedal ganglion; Mz, mesoderm cells. 



* Vide especially Loven, 

 " Bidrag till Kannedomen 

 om Utvecklingen af Mol- 

 lusca Acephala Lamelli- 

 branchiata." Stockholm, 

 1848. 



Flemming, "Studien iiber die Entwickelungsgeschichte der Najaden." 

 Sitzungsler. der K. Akad. der Wissensch. Vienna, 1875. 



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



B. Hatschek, "Ueber die Entwick-gesch. von Teredo." Arbeiten aus den 

 tool. Institute, etc., Tom. III. Vienna, 1881. 



