. Text-book of embryology. Embryology. IX MOLLUSCA 353 hoUowB out to form the kidney, whilst a band of cells grows out from each and forms a ring round the gut, splits into two layers, and forms the pericardium. The accounts of Harms and Lillie leave no doubt in tlie mind that in the embryo of the Unionidae we are merely dealing with an ordinary Pelecypod veliger modified for a parasitic Fig. 284.—Transverse section of a Glochidium larva of Unio which is already fixed in the tissues of its (After Harm.) a,m, cells which will form the adult mantle ; int, intestine: ^.m, vacuo
. Text-book of embryology. Embryology. IX MOLLUSCA 353 hoUowB out to form the kidney, whilst a band of cells grows out from each and forms a ring round the gut, splits into two layers, and forms the pericardium. The accounts of Harms and Lillie leave no doubt in tlie mind that in the embryo of the Unionidae we are merely dealing with an ordinary Pelecypod veliger modified for a parasitic Fig. 284.—Transverse section of a Glochidium larva of Unio which is already fixed in the tissues of its (After Harm.) a,m, cells which will form the adult mantle ; int, intestine: ^.m, vacuolated cells of larval mantle, which .absorb food material from the host; m,c, mantle-cavity ; p.£r, rudiments of pedal ganglit; t, teetli of larval shell. CEPHALOPODA—Loligo, Sepia We must now turn to the study of the embryology of the highest MoUusca, the Cephalopoda (lit. head-footed), so called because the fore part of the foot has grown into a frill surrounding the head. Two genera are represented by common species, both on the English coasts and in the Mediterranean; these are Sepia officinalis and Loligo vulgaris. So far as is known, the development of both pursues a practically identical course. We shall select- Loligo as "a type for special study, because its development has been more completely worked out, and because species of this genus are common on the American coast; but we shall not hesitate to fill up lacunae in our . knowledge of the development of Loligo by the description of corre- sponding stages from the development of Sepia, when these are better known. The eggs of both genera, like all Cephalopod eggs so far described, contain an abundance of yolk, the cytoplasm being mainly restricted to a small disc at the animal pole of the egg, in which the nucleus is VOL. I 2 a. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may no
Size: 1737px × 1438px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
