14 



MOLLUSCA. 



poda). They usually have the form of sacs with a wide lumen, 

 and open on the one hand into the body cavity (pericardial 

 sinus), and on the other to the exterior by a lateral opening. 



In all probability the mol- 

 luscan kidney is homologous 

 with an annelidan segmental 

 organ. The internal, funnel- 

 shaped opening is frequently 

 beset with cilia. The unus 

 is very often removed from 

 the middle line, and placed 

 on one side of the body. 



A compact heart is always 

 present, driving the blood 

 through the vessels into the 

 organs. The vascular system 

 is never completely closed, 

 for, even when the artories 

 and veins are connected/ by 

 capillaries, blood sinuses de- 

 rived from the body carity, 

 are inserted into the course 

 of the vessels. The heart is 

 always arterial i.e., systemic 



\| and receives arterial blood 



\ from the respiratory organs. 



Respiration is in all eases 

 carried on through the 

 general outer surface of the 

 body ; but in addition special 

 respiratory organs, in the 

 form of branchice, more 

 rarely of lungs, are present. 

 The branchiae are ciliated 

 projections of the body sur- 



FIG. 497. Nervous system of Cassidaria (after 

 Haller). Cg, cerebral ganglion; P ff , pedal face, and are usually placed 



ganglion; pig, pieurai ganglion; Bg, buccai between the mantle and the 



ganglion; Gsp, supramtestmal ganglion; Gsb, 



Bubintestinal ganglion; Vg, visceral ganglion; f OOt ; they may have the 



*' otocyst - form of branched append- 



ages, or of broad lamellae (Lamellibranchiata). The lung, on the 

 oth.er hand, is derived from the mantle cavity, which is filled with 



