THE LAMELLIBRANCHIA 



245 



in the cloacal or suprabranchial chamber, as in Cardium and several 

 other incubatory forms, or in the oviduct itself in Ostraea edulis. 



The formative pole of the ovum is opposite to the micropylar 

 end. The segmentation is unequal from the first cleavage onwards. 

 The macromere formed at the first cleavage is loaded with yolk 

 granules and remains single for a long time, but gives rise to the 

 three first groups of micromeres, which partly cover it as with a 

 cap (Fig. 9, C). Finally, the macromere divides to form the endo- 

 derm cells. The gastrula is rarely formed by invagination (Pisidium, 

 Kay Lankester), but in nearly all cases by epiboly, or sometimes 

 by a process midway between the two, in which there is at first 

 an epiboly resulting from the multiplication of the small ectodermic 

 cells surrounding the single macromere, and finally an invagination 

 after division of the macromere. This process is found in Ostraea, 

 Cyclas, and the Unionidae, and in the two last-named the segmenta- 

 tion cavity is very large and the enteron small (Fig. 227). The 

 blastopore remains open in some cases, e.g. in Ostraea, but closes in 

 Cyclas, Pisidium, the Unionidae, Dreissensia, Teredo, etc. But the 

 mouth and oesophagus are soon formed by a secondary ectodermic 

 invagination at the point of closure of the blastopore. The' yolk 

 remains in connection with the dorsal surface of the enteron and is 

 slowly absorbed. The endoderm gives rise to the stomach and the 

 two liver lobes and to the intestine ; the liver lobes often display 

 a marked asymmetry, the left lobe being larger than the right in 

 Mytilus, Dreissensia, and Yoldia. The anal ectodermic invagination 

 placing the intestine in communication with the exterior is gener- 

 ally at the extreme posterior end of the embryo, is very short, and 

 very late in appearance. The mesoderm originates, at an early 

 period, from the most posterior of the four primary endoderm 

 cells ; the resulting mesomeres take up a position between the 

 ectoderm and endoderm in the form of two symmetrical mesoderm 

 bands. 



In its general characters the development of the Lamelli- 

 branchia conforms to the type observed in the other classes of 

 the Mollusca, but a certain number of special features must be 

 noted. (1) The shell- gland makes its appearance at an early 

 period in the normal position ; that is to say, at the formative 

 pole, nearly opposite to the blastopore (Fig. 223, sk). It is single, 

 like the shell-gland of all other Molluscs. During its extension it 

 gives rise to a saddle -shaped cuticular pellicle, which becomes 

 calcified at two symmetrical points, right and left of the middle 

 line. These two centres of calcification eventually form the two 

 valves of the shell, but, except in the Unionidae, they do not 

 develop as fast as the subjacent lobes of the mantle. The two 

 valves remain united by the median and dorsal part of the primitively 

 single shell, and the ligament is formed at this line of union. 



