384 



OILMAN A. DREW. 



Alimentary Canal. — The primitive gut is formed by the 

 separation and division of cells on one side of the embryo. 

 It is carried further into the interior by the addition of cells 

 around the blastopore. These cells form the stomodseum. 

 Later the gut grows posteriorly, beneath the shell-gland, 

 and forms the stomach and intestine. The anus opens into 

 the mantle chamber near the anal pore in the test. The 

 future shape of the intestine seems to depend upon the posi- 

 tion of certain organs during its elongation. (See pp. 347 — 

 353, and figs. 8, 9, 11, 15, 24, 25, 34, 36, 40, 45, 46, 47, and 

 48, and Text-figs. M to S.) 



Labial Palps. — The labial palps are marked out as 

 patches of cilia about the time that the third lobe of the gill 

 begins to form (fig. 41). The ciliated patches along the sides 

 of the body are bent so as to form grooves (fig. 62, Ip.) ; the 

 dorsal portions of the patches form the outer palps, and the 

 ventral portions the inner palps. The palp appendages are 

 formed by unequal growth of the posterior portion of the 

 outer palps, and each corresponds morphologically to a pair 

 of ridges with a groove between them. They can be ex- 

 tended beyond the margins of the shell, and they are used as 

 food collectors. (See pp. 353—357, and figs. 41, 45, 47, 48, 

 54, 55, 56, 57, 58, 59, 60, 62, and 66.) 



Grills. — The gills are formed as folds on the inner sides of 

 the lobes of the mantle. The folds form lobes that grow to 

 form filaments and finally plates. The inner plates are 

 formed first. The outer plates are formed by growth at the 

 bases of the inner plates. A study of their development 

 throws no light on the phylogeny of the gills. (See pp. 

 357_363, and figs. 39, 40, 41, 45, 48, 52, and 53.) 



Hypobranchial Glands. — The hypobranchial glands 

 are formed about the time that the animals become sexually 

 mature. They seem to furnish the secretions from which 

 the brood-sac is formed, and they may have other functions. 

 (See pp. 363—365.) 



Pericardium. — The pericardium is a remnant of a cavity 

 that probably represents a schizocoele. Its epithelial lining 



