42 



Its position is shown in fics. C a.u 'J ; ana its extent and 

 structure in fig. 22, which repi-esents a transvorse sec- 

 tion of the whole f^land in a specimen about a half centi- 

 meter long. The gland consists of numerous^ more or less 

 spherical, vesicular acini whose average diameter is about 

 a fortieth millimeter. They are lined by flattened, non- 

 ciliated, slightly granular cells. From the gland a duct 

 passes posteriorly to open on the dorsal outer surface. 



This gland appears in the young "Ship-worm" soon af- 

 ter attachment as a single median small vesicle, of appa- 

 rently epidermal derivation. As the animal grows , new ves- 

 icles are formed as outgrov/^ths from those already present. 

 What the function of the gland is, iias not been determined, 

 but its position indicates that it may be the secretion of 

 a poison noxious to enemies that may get into the end of 

 the "Ship-v^orm" bui'row. 



e. The Muscular System. 



Early in this century it was one oJ' the mont debat- 

 ed questions in Zoology, whether the imiscle then known in 

 the "Ship-v/orms" was liomoloj-ous witli the anterior or pos- 



