THE ALIMENTARY CANAL. 67 



tts relation to the other ontogenetic processes, have already been 

 described in a former section (p. 30). The ectodermal invagination 

 yields the oesophagus; the stomach, liver and intestine are ento- 

 dermal. The anus, in the majority of cases observed, seems to have 

 been formed by direct fusion of the entoderm with the ectoderm, so 

 that the posterior part of the intestine would be entodermal ; in 

 Teredo, however, there is, according to HATSCHEK, a proctodaeal in- 

 vagination, and a similar invagination is described by VOELTZKOW 

 as occurring in Entovalva (No. 57). 



The further development of the intestine consists in its increase 

 in length, as a result of which it becomes coiled. A circular con- 

 striction marks off the stomach from the intestine. As early as the 

 Trochophore stage, a pair of sac-like outgrowths appear in connection 

 with the stomach; this is the rudiment of the liver (Fig. 16, p. 33) 

 with which the yolk-laden remains of the macromeres become incor- 

 porated (Fig. 18, p. 36). A peculiar phenomenon in connection with 

 these two liver-sacs, which at first are spherical, is the occurrence of 

 rhythmical movements ; these are no doubt to be traced back to the 

 action of mesoderm-cells which have become apposed to the entoderm 

 wall (LovE*N, ZIEGLER). The passages from the liver into the 

 stomach which at first are wide, become narrow later and form the 

 efferent ducts ; the bulgings found on the liver-sacs mark its separate 

 lobes and lobules (Fig. 31, /, p. 75). 



In the stomodaeum of Cardium, LOVEN observed a small bulging of the 

 ventral wall which involuntarily recalls the radula-sac of other Molluscs, an 

 organ which is known to be wanting in the Lamellibranchia. It cannot be 

 connected with the crystalline style-sac, as this is invariably an entodermal 

 derivative. The sac which contains the crystalline style is formed as an out- 

 growth of the wall of the stomach. This structure which, as has long been 

 known, occurs also in the Gastropoda, appears, according to the most recent 

 view, to yield a secretion (the crystalline style) which serves for enveloping 

 solid particles of food, and thus protects the wall of the intestine (BABBOIS). 

 No statements as to the ontogenetic formation of the crystalline style-sac 

 are known to us. 



The layer of muscle and connective tissue which forms the outer 

 wall of the intestine is yielded by the mesoderm-cells distributed in 

 the primary body-cavity, which become applied either to the ento- 

 derm or to the ectoderm. 



