The royal natural history . has taken a great place in zoologyin recent years, since the Russian naturalist,Kowalevski found that many different classes of animals, in developing from theegg, passed through such a stage. Haeckel, generalising from these facts, inventedhis Gastrea theory, according to which all animals in which the gastrula stageoccurs must have been descended from a common primitive form, Gastrea, whichlias, however, in its simplest form long been extinct, but of which the Coelenteratesare the closest modern representatives. The gastrula of Monoxenia is of the simplest kind, t
The royal natural history . has taken a great place in zoologyin recent years, since the Russian naturalist,Kowalevski found that many different classes of animals, in developing from theegg, passed through such a stage. Haeckel, generalising from these facts, inventedhis Gastrea theory, according to which all animals in which the gastrula stageoccurs must have been descended from a common primitive form, Gastrea, whichlias, however, in its simplest form long been extinct, but of which the Coelenteratesare the closest modern representatives. The gastrula of Monoxenia is of the simplest kind, the infolding beingcomplete, and the larva forming a sac, whose walls consist of two layers of cells,or germinal layers, an outer ectoderm and an inner endoderm (see section given intlic illustration). The transition from the flat dish-shape (H) to the sac with anarrow mouth is at once clear, and the knowledge that all the Coelenterates proceedfrom a similar larva, and that all the complications of their various systems are. Monoxenia darwini (highly magnified). SEA-ANEMONES AND CORALS. 497 developed from such a simple gastrula, throws much light on their anatomy. Duringthese transformations, the endoderm, whose cells multiply, continues as an uninter-rupted lining to the stomach and its appendages, while the ectoderm yields theconstituents of the skin. A third intermediate gelatinous layer, the mesogloea, arisesbetween the outer and innerlayers; in this, muscles andconnective interstitial tissuesappear. The chief part of thejelly forming the great umbrellaof the DiscomedussB consists ofthis mesoglsea. In the mesoglseaof one division of the corals thecalcifications take place. Theseinternal calcifications play, how-ever, but a very small part inthe great rock-making activitiesof corals as a whole, the mostimportant calcifications beingex-ternal. Returning to Haeckelsaccount of Monoxenia, althoughthe transition from the gastrulalarva to the adult animal hasnot been observed, ther
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
