THE LIFE-HISTORY OF NUOtJLA DELPHJNODO^TA. 383 



been traced. The following is a brief review of the organs 

 of Nucula delphinodonta. 



Test. — The test consists of five rows of flattened cells, 

 that together cover the greater portion of the body of the 

 embryo. The cilia on the test are short and evenly dis- 

 tributed. The size and distribution of the cilia are probably 

 the result of the protection afforded the developing embryo 

 by the brood-sac. The test is finally thrown away. It is 

 probably homologous to the velum of most Lamellibranch 

 embryos. (See pp. 335—339, and figs. 1 1, 24, 2b, 34, and 35.) 



Apical Plate. — The apical plate is quite extensive, and 

 bears short diffuse cilia that resemble the cilia on the test 

 cells. The size of the apical cilia is probably also the result 

 of the protection afforded the developing embryo by the 

 brood-sac. The apical plate is thrown away with the test. 

 (See p. 339, and figs. 11 and 24.) 



Shell. — The shell begins to form some time before the 

 test is shed. The prodissoconch has a rounded outline and a 

 . short straight hinge-line. The adult shell is very robust. 

 (See pp. 339—341, and figs. 20, 36, 50, and 51.) 



Mantle. — The mantle lobes are formed by the growth 

 and folding of the shell-gland. There are no tentacles on 

 the margins of the mantle, and no siphons are formed. (See 

 pp. 341, 342, and figs. 8, 17, 20, 48, and 69.) 



Foot. — The foot is formed by the growth of tissue that, 

 at first, lies between the stomodseum and the gut. At the 

 time the test is shed it is very small and cannot be moved. 

 The side flaps are developed as the result of unequal growth 

 of the ventral side of the foot. The foot is a remarkably 

 good burrowing organ, and it seems never to be used in 

 creeping. (See pp. 342—346, and figs. 25, 28, 34, 36, 39, 40, 

 41, 48, 49, and 69.) 



Byssal Gland. — The byssal gland is formed as an in- 

 vagination on the ventral surface of the foot soon after the 

 test is shed. It becomes very extensive, but in the adult is 

 quite small. It seems never to form fibres. (See pp. 346, 

 347, and figs. 39, 40, 41, 45, and 48.) 



