62 



g. The Circulatory System. 



The circulatory system of the "Ship-v^orms" is pecu- 

 liar in relation to tlie peculiar form of the body. The 

 growth of the visceral mass ventrally at first, and after- 

 wards its {jreat elongation posterially along with the 

 elongation of the rest of the body accounts for the chan- 

 ges tliat have taken place. Doubtless the ancestors of Te- 

 redo were Lamellibranchs with typical circulation, in 

 which on either side in the pericardial cavity lay an aviricle 

 latent to, and emptying into, the median ventricle vrhich 

 surrounded the intestine. . From the ventricle the ante- 

 rior aorta passed forv/ard above the intestine and the pos- 

 terior aorta backward below the intestine. In Teredo, 

 the pericardial space, v/ith its contained parts, has come 

 to lie on the morphologically ventral side of the intes- 

 tine, and the relations of the various parts of the circu- 

 latory system to each other have been radically changed. 



The youngest stage of the circulatory system I have 

 observed in detail is in specimens 2 rnm. long. Here the 

 heart consists of two almost separate halves (fig. 52). On 

 either side a more or less spherical auricle (an. ) lies 

 lateral and slightly vential to, and loads into, a more or 



