152 



TEREDINIDjE. 



the homologue of the sinuated portion of the pallial mus- 

 cle in Pholas. In both cases it is placed at the base of 

 the tubes or siphons. 



4. Embryology. Nearly all our knowledge of this 

 part of the natural history of Teredo is derived from an 

 elaborate memoir by Quatrefages in the ' Annales des 

 Sciences Naturelles ' for 1849. The process of oviposi- 

 tion is successive and of long duration. During a 

 period which varies according to the species, the female 

 emits her eggs, which are arrested and lodged in the 

 folds of the respiratory organs. In this singular nest 

 they are fertilized by the spermatozoa of a male, 

 disseminated through the mass of the surrounding 

 water, some of which find their way into the bran- 

 chial tube of the female, where they meet with the 

 eggs and vivify tnem by contact. The same me- 

 thod of impregnation takes place in Anodonta or the 

 freshwater mussel. The egg, while in the ovary, 

 consists at first of an extremely minute globule, which 

 is simple, homogeneous, transparent, and quite colour- 

 less. This is called " the vesicle of Purkinje." Some 

 very fine granules soon appear in the substance of this 

 globule ; and in a short time may be seen developed in 

 its interior a second globule called " the germinative 

 spot of Wagner." The two globules increase together 

 for some time before the formation of the yelk-mem- 

 brane which covers the whole. In this state the egg is 

 exactly spherical. Its volume then becomes enlarged ; 

 and after passing through other phases, it assumes 

 the shape of a tear, and when emitted the sphere is 

 converted into an irregular oval. The spermatozoa 

 now attach themselves to the egg, and certain internal 

 movements and changes ensue. These last for about 

 two hours ; the yelk-gramiles are distributed through- 



