22 INTRODUCTION. 



aerated and nutrient jfluids of the pallial sinuses, escape at a certain stage of that develop- 

 ment, by dehiscence, at or near the free cihated border of the palhal lobes. The course 

 and phenomena of this development would form a most interesting and acceptable subject 

 to a competent microscopical observer favorably situated for pursuing it throughout the 

 breeding season of the TerehratulcB. 



I subjoin a few stages observed in ova taken from the ramified ovarium of a Lingula, 

 preserved in spirits. 



PI. 1, fig. 7, a, is an impregnated ovum, in which the germinal vesicle or vesicles have 

 disappeared, and the germ-mass has been formed, occupying the entire ovum, which has 

 assumed an oblong form, a peripheral stratum of the derivative germ-cells was more com- 

 pact and of a somewhat lighter colour than the central mass. 



h, shows the formation of a smooth membrane, probably covered by ciliated epithelium, 

 around the germ-mass. 



c, is a transverse section of such an ovum showing its triedral figiu*e. 



d, is an ovum further advanced, with the rudiment of a peduncle. 



e, an embryo with the peduncle more produced. I could distinguish no organs in 

 these embryos of the Lingula -. there was no trace of shell. 



The dissections from which Mr. Scharf's drawings, engraved in the subjoined plates, 

 were taken, were prepared by my former anatomical assistant, Mr. Henry Goadby, to 

 whose skill in the manipulation of minute objects I am more especially indebted for the 

 demonstration of the nervous system, as shown in figs. I and 2, pi. 2. 



