[207] A secondary larval form is less likely to be repeated in development than an ancestral adult stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain, to drop out by the occurrence of a reversion to the original type of development.
[208] Bobretzky first stated that the invagination remained open, but subsequently corrected himself. Zeit. f. Wiss. Zool., Bd. XXIV. p. 186.
[209] Reichenbach describes these cells, and states that there is a thickening of the epiblast adjoining them. In one place he states that the heart arises from this thickening of epiblast, and in another that it arises from the mesoblast. An epiblastic origin of the heart is extremely improbable.
CHAPTER XIX.
PŒCILOPODA, PYCNOGONIDA, TARDIGRADA, AND LINGUATULIDA; AND COMPARATIVE SUMMARY OF ARTHROPODAN DEVELOPMENT.
The groups dealt with in the present Chapter undoubtedly belong to the Arthropoda. They are not closely related, and in the case of each group it is still uncertain with which of the main phyla they should be united. It is possible that they may all be offshoots from the Arachnidan phylum.
Pœcilopoda.
The development of Limulus has been studied by Dohrn (No. [533]) and Packard (No. [534]). The ova are laid in the sand near the spring-tide marks. They are enveloped in a thick chorion formed of several layers; and (during the later stages of development at any rate) there is a membrane within the chorion which exhibits clear indications of cell outlines[210].
There is a centrolecithal segmentation, which ends in the formation of a blastoderm enclosing a central yolk mass. A ventral plate is then formed, which is thicker in the region where the abdomen is eventually developed. Six segments soon become faintly indicated in the cephalothoracic region, the ends of which grow out into prominent appendages ([fig. 245] A); of these there are six pairs, which increase in size from before backwards. A stomodæum (m) is by this time established and is placed well in front of the foremost pair of appendages[211].