394 PROCEEDINGS OF THE ACADEMY OF [April, 



present. The stomodceum later forms at the point where the blasto- 

 pore closed. 



The shell-gland at first forms a deep invagination, which later opens 

 out and covers the posterior end of the veliger with a cap of large 

 cells which soon begin to secrete the shell. From the first the shell 

 is slightly shifted toward the left, and this asymmetry becomes more 

 marked with continued growth. With the enlargement of the shell 

 a conspicuous posterior vesicle results. 



The foot arises as an unpaired swelling below the stomodceum. Its 

 under surface later secretes an operculum. 



The first velar row is formed from the anterior trochoblasts (A and 

 B quadrants), the tips of the anterior arm of the cross, and possibly 

 from other cells of the first quartet in this region. The second velar 

 row is derived from underlying cells of the second quartet. A post- 

 oral velar area is but slightly marked. In later development the velum 

 becomes bilobed and broadly expanded. 



A prominent head vesicle is not present in the older vehgers, and 

 with this may be correlated the development of a large posterior vesicle. 

 No apical sense-organ has been found, nor are distinctly marked apical 

 plates present. The cerebral gangha appear in the angles between 

 the anterior and lateral arms of the cross. Otocysts are formed by 

 invaginations of the ectoderm upon the sides of the foot, and pedal 

 gangha appear closely associated with them. The eyes are late in 

 appearing and are intimately connected with the rudiments of the 

 cerebral ganglia. 



The anal kidney of the larva is derived from the ectoderm, coming 

 from 3c^"^ and associated cells. With the torsion of the larva this 

 group is shifted farther to the right, and eventually lies well up on the 

 right side of the veliger above the anal opening. Primitive ex- 

 cretory cells are also found lying in the body cavity laterally behind 

 the velum. 



The enteron is formed by invagination of the entomeres, which at 

 first form an elongated sac; with the evagination of the shell-gland 

 this becomes rounded. The hver is derived from large yolk-ladened 

 cells lying at the anterior end of the enteron, and later the rudiment 

 of this organ becomes turned toward the left side. Torsion of the 

 enteron results from lengthening of the left side and is caused by 

 increased growth of that region. The intestine is at first a solid thick 

 cell-strand and is composed largely of entoblasts from 4d; it later 

 elongates and acquires a lumen. 



