42 



MOLLUSCA. 



the penis is remote from the sexual opening, a ciliated furrow ) 

 present, which conducts the spermatozoa from the opening to tne 

 penis (Murex, Dolium, Strombus). 



The embryonic* development begins with an unequal segmenta- 

 tion leading to the formation of a blastula or gastrula. Later the 

 embryo acquires a ciliated velum, the first rudiment of the shell, foot, 

 and primitive kidney, and rotates in the fluid albumen of the egg 

 by the vibrations of the cilia. 



The free development is either direct, the just -hatched anima 

 possessing (excepting for the rudiments of larval organs) the form 

 and organization of the adult (Pulmonata), or it takes place by a 

 metamorphosis. Almost all marine Gastropoda develop by meta- 



A 



Fia. 518. Some stages in the embryonic development of Planorbis (after C. Rabl). a, 

 optical section through a segmenting ovum (24 segments). ItJc, polar bodies; Fh, 

 segmentation cavity, b, stage with four mesoderm cells, viewed from the vegetative 

 (lower) pole. Ms, mesoderm cells ; En, endoderm ; .Ec, ectoderm, c, Oblique optical 

 longitudinal section through the stage with four mesoderm cells, d, Older embryo, in 

 which the shell gland has shifted to the right. Sdr, shell gland ; S, shell ; O, mouth ; 

 -D, alimentary canal ; It, commencing radula ; Sp, apical plate (thickening of praeoral 

 lobe) ; Oc, eyes ; Ot, otolith ; N, primitive kidney ; Ve , velum. 



morphosis, and the larvae possess two large ciliated sails (velum), which 

 serve as locomotory organs in place of the still rudimentary foot. 

 The shell, which is already present on the dorsal surface, is still small 

 . and flat with hardly any trace of the spiral twisting, and can usually 

 be closed by an operculum which is attached to the foot. Very of ten 

 a change of shell is effected, the old embryonic shell being thrown ofi 

 and a new one formed in its place. 



* Cf. especially. 



N. Bobretzky, i( Studien liber die embryonale Entwickelung der Gastropoden/' 

 Archiv fwr mik. Anat., Tom XIII. , 1876. 



C. Rabl, "Ueber die Entwickelung der Tellerschnecke." Jlforph. JalirK 

 Tom V. 



Also Fol, Butscbli, E. Lankester, etc. 



