MOLLUSCA. 



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

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

 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 anus 

 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 arteries 

 and veins are connected by 

 capillaries, blood sinuses, de- 

 rived from the body cavity, 

 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 cases 

 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; Pg, pedal face, and are usually placed 



ganglion; pi ff , pieurai ganglion; Bg, buccai between the mantle and the 



ganglion; Gup, supraintestinal ganglion; Gso, 



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



form of branched append- 

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

 other hand, is derived from the mantle cavity, which is filled with 



