276 



SWAN MUSSELS. PHYLUM MOLLUSCA 



-po. 



azL. 



rnu 



— p-r. 



— p.CLd^ 



visceral hump, but the gonads and intestine lie in the soft region 

 of the foot. The mcnith leads into a gullet, which passes upwards 

 into a moderate-sized stomach situated behind the anterior 



adductor muscle. Into the 

 stomach opens by several ducts 

 a series of digestive diverticula, 

 often miscalled a liver ; small 

 particles are circulated through 

 these by cilia, taken up by the 

 cells, and digested intracellu- 

 larly. The hinder end of the 

 stomach communicates on the 

 right side with a closed groove 

 of the intestine, the caecum, 

 which contains a transparent, 

 gelatinous rod, known as the 

 crystalline style (Fig. 204). 

 This is composed of a protein 

 substance and projects into 

 the stomach, where it rotates 

 and dissolves. It liberates an 

 amylase which digests carbo- 

 hydrates ; this is the only 

 extracellular digestive enzyme 

 produced by the mussel. The 

 intestine (Figs. 205, 206) starts 

 from the lower side of the 

 stomach, takes several coils in 

 the soft upper part of the 

 foot, turns upwards, and runs 

 straight backwards in the 

 middle line of the upper part of the body to the anus. The 

 straight part of the intestine is known as the rectum. It lies 

 in the pericardial cavity surrounded by the ventricle of the heart. 

 The ventral wall of the rectum is folded to form a longitudinal 

 ridge or typhlosole. Almost the whole movement of the food 

 in the gut is carried out by cilia. 



^ ---c?.^ 



Fig. 203. — Part of the dorsal side of a 

 swan mussel in which the peri- 

 cardial cavity has been opened. 



ail.. Auricle ; d.s., margin of dorsal siphon ; g., 

 hinder tips of gills, fused to form floor of 

 cloacal chamber ; p. ad., posterior adductor 

 muscle ; pr., posterior retractor muscle ; rm., 

 rectum ; rp.o., renopericardial opening ; v., 

 ventricle ; v.s., margin of ventral siphon 

 (opened out by spreading the mantle). 



Note between the posterior adductor muscles the 

 fusion of the mantle edges for a short distance, 

 roofing in the cloacal chamber just above the 

 dorsal siphon. 



