BY PROFESSOR W. A. HASWELL. 511 



rudiment of the nervous system becomes formed. This appears 

 as a bridge of finely fibrillated material (fig. 27) running trans- 

 versely oyer the pharynx and surrounded by numerous cells — the 

 rudiment of the brain. This is well established when the pharynx 

 is still a narrow tube with a thin wall enclosing a core of cells. 

 From the brain two narrower strands of similar material run 

 backwards at the sides, each surrounded by groups of cells; and 

 are traceable for some distance through the body, though not 

 extending at first very far back. Before long each longitudinal 

 cord bifurcates to form a dorsal and a ventral branch. About 

 the same time a nerve is formed running forwards from the brain 

 towards the anterior extremity. 



The tail (figs. 29-31) appears first, when the embryo is about 

 0-09 mm. in length, as a rounded process consisting of an 

 undifferentiated mass of cells covered by a continuation of 

 the thin investing layer. While still very short it becomes 

 bilobed at the extremity. Before this takes place internal 

 differentiation has begun. A very regularly arranged line of 

 small nuclei becomes ranged along each side below the investing 

 layer; and the rest of the tissue forms a central core. As the two 

 branches grow out at the bilobed extremity, their constituent cells 

 take on a similar arrangement. The central core becomes divided 

 into four longitudinal rows of cells — two dorsal and two ventral; 

 these are the myoblasts. They become enlarged, and their nuclei 

 become vesicular. In the substance of their protoplasm external 

 to the nuclei appear bright-looking longitudinal bands which 

 stain deeply with eosin; these are the first-formed muscular fibres. 

 They soon become more numerous, and as the tail becomes fully 

 developed, take the form of the four longitudinal bands. The 

 small cells of the lateral rows, at first placed close together, 

 become separated from one another by distinct intervals as the 

 tail elongates, and become the lateral series of small cells referred 

 to above in the account of the tail of the adult. 



The Sporocysts and Cercari^ of a Species of Gasterostomum. 



Only one specimen of the mussel was observed to be infested 



with this parasite. It presented a very remarkable appearance. 



