138 FRESH- WATER MUSSEL -COMMON MARINE MUSSEL 



Open the ureter and the kidney proper, and note that they are m com- 

 mtinication with each other at the posterior end. 



Note that the vena cava, which carries impure blood to the gills, passes 

 between the kidneys and through their walls. 



The gonads (ovaries or testes), when ripe, form the greater part of the 

 visceral mass. 



A. The Alimentary System. 



Carefully remove the digestive gland so as to expose the stomach. Expose 

 the intestine by carefully cutting away as much of the visceral mass as is 

 necessarj'. 



The mouth lies behind the anterior adductor muscle and between the labial 

 palps. It leads into a short and straight oesophagus which passes almost 



Pecicardium 



■, Dorsal retractors 



Cerebro-pedal „ , , '"^^"PL-l^^^^teJi'"' • ''"''"'^ 



connective redai 8*?S"oi^5^^ » ' "•/^'^l^^^sLr''™''''''"'"" 

 CEsophagus \SjomachU((jJ^f\ ;.■'■*■ "'7 I \ /^^^^^^*?^Ur?ter 

 Anterior'' '■ j^^^f\ii^^^^''^'y^'2kt^^^^^''^^ ^^^^^^. ._Posterior 



^,, Posterior 

 'M adductor 



Labial 

 palps 



°°'^?JS^r^'^«^«''-- M-t<e cerebro-visceral 



connective 



Fig. 47. — The Marine Mussel [Mytilus) dissected' from the left side to show the 



general strncture. 



vertically into the stomach. The stomach may contain a crystalline style. 

 The intestine arises from the lower wall of the stomach and passes backwards 

 and down into the visceral mass, where it coUs upon itself; it then ascends 

 parallel to its first portion, turns sharply back and proceeds as the rectum 

 through the pericardium, traversing the ventricle, and above the posterior 

 adductor muscle, to end in the anus situated in the exhalent siphon (or 

 cloaca). The ventral wall of the rectum is folded into a longitudinal ridge 

 or typhlosole. 



\In Mytilus the mouth lies between the labial palps. From it the oesophagus, 

 which is flattened dor so-ventr ally, leads into the stomach. From the stomach the 

 intestine is continued backwards to the region between the dorsal retractors. There 

 it bends sharply upon itself and coils forwards in the digestive gland, over the 

 stomach, and thence passes backwards through the heart as the rectum, which opens 

 at the anu^ situated above the posterior adductor muscle. At the posterior end of 

 the digestive gland the intestine gives rise to a diverticulum, which passes back- 

 wards under the floor of the pericardium, to end blindly jv^t under the anus. This 

 diverticulum usually contains the crystalline style, a gelatinous rod of reserve food 

 7naterial.] 



