. Cephalopoda. Cephalopoda. 89 The shell ofOrthoceras can be reconstructed according to the young- est shells (Pocta, 1902). At any rate, the embryonic chamber and a living chamber formed by the adjacent part of the shell must have been present at the time of hatching. Nothing is known on the development of the siphuncle and prosiphuncle, which is quite obscure in Nautilus and Spirula and too modified atypically in Sepia to permit any direct conclusions. The prosiphuncle is formed by a part of the shell epithelium which remains fixed long after the rest has become detached from the inner side
. Cephalopoda. Cephalopoda. 89 The shell ofOrthoceras can be reconstructed according to the young- est shells (Pocta, 1902). At any rate, the embryonic chamber and a living chamber formed by the adjacent part of the shell must have been present at the time of hatching. Nothing is known on the development of the siphuncle and prosiphuncle, which is quite obscure in Nautilus and Spirula and too modified atypically in Sepia to permit any direct conclusions. The prosiphuncle is formed by a part of the shell epithelium which remains fixed long after the rest has become detached from the inner side of the embryonic shell. The siphuncle develops as a process of the posterior part of the body, secreting a hardening but not calcified shell substance even before the formation of the first septum,"= while the space created is filled with gas.''"'' These are the preconditions for the formation of the first septum with the primary caecum. Further development involves: 1) a continuous or periodic growth at the free shell margin (ostracum); 2) the deposition of thickening strata (hypostracum) before the annulus; 3) the formation of new septa and septal necks behind the annulus, after the posterior end of the animal had moved some distance forward in the shell tube, leaving behind each time only the beginning of the siphuncle; 4) the formation, by the annulus, of a thin layer which separates the wall from the septa and which belongs to the FIGURE 22. Hypothetical development of Or thoc eras (about 6 x natural size): a) - embryo before the formation of the first septum; b) - embryo with one chamber (embryonic chamber); c) — median section through the posterior part of the body of (a); d) — median section through the posterior part of the body of (b); e) - formation of the second septum with the corresponding septal neck. 1 — shell apex (initial vesicle); 2 — shell epithelium; 3 — conical part of the shell; 4 - mantle margin; 5 - anus; 6 - mantle cav
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectcephalopoda
