II ONTOGENY OF CNIDABIA 129 



It is important to note that in most of the higher bilaterally symmetrical animals 

 the bilateral symmetry appears very early. The gastrula of these animals is 

 bilaterally sjrmmetrioal, i.e. only one plane can be made to cut it into two exactly 

 similar halves. We can consequently distinguish in the gastrula upper and lower, 

 anterior and posterior, right and left. The gastrula of the radiate OceUnterata, on 

 the contrary, is radiate or rather uniaxial. There is one principal axis round which 

 all the elements of the body are arranged in circles. This principal axis has unlike 

 poles ; at one pole, the vegetative, lies the blastopore ; the other, the animal pole, is 

 closed. 



Bilateral symmetry is shifted back in many bilaterally symmetrical animals 

 to much younger stages of development — to the blastula or the segmentation stages. 

 In a few cases even the egg is bilaterally symmetrical, and the position of the future 

 principal regions of the body can be determined even in it. 



In the uniaxial gastrula the blastopore is round and closes to a point. In 

 the bilateral gastrula, however, it has become slit-like, and closes either from front 

 to back or vice versa, in a line lying in the plane of symmetry or median plane of 

 the body. 



Ontogeny of the Cnidaria. — The segmentation is everywhere complete. The 

 formation of the gastrula occurs by invagination, epibole, or delamination. In the 

 last case a cceloplanula arises direct. Where a ccelogastrula or a sterrogastmla 

 occurs it changes, in all cases except the Gtenophora, into a planula by the closing 

 of the blastopore. This is generally free-swimming and ciliated, and has a tuft of 

 long, mostly immobile sensory hairs at the original animal pole, which we now call 

 aboral. A Sydroid arises out of the blastula by the formation of the definite oral 

 aperture by means of a breach where the blastopore closed, the animal having 

 attached itself by the aboral end of the body. Round the mouth, the tentacles bud 

 out as hollow outgrowths of the ectoderm and endoderm. 



The direct development of a Oraspedote Medusa from the fertilised egg is best 

 known in Geryonia, with whose blastula we are already acquainted. 



Between ectoderm and endoderm a jelly is formed, which constantly increases in 

 mass, so that the ectoderm sac is separated by a great interval from the endoderm 

 sac which it encloses. At one point only, the future oral pole, which probably agrees 

 with the vegetative pole, the endoderm sac remains in contact with the ectoderm 

 sac. The permanent mouth is formed at this point of junction by means of a breach, 

 while at the same time, at some distance from the mouth, the velum arises as a 

 circular thickening of the ectoderm, and the 6 tentacles as buds, into whose axes 

 solid processes of the endodermal sac grow. The connection of the endodermal ten- 

 tacle axis with the gastral sac soon ceases. The oral surface of the larva, which is 

 surrounded by the tentacles and the velum, sinks in and becomes the concave sub- 

 umbrella. The Medusa form thus gradually comes into existence. How the radial 

 vessels are formed in Geryonia has not been investigated. 



A Scyphopolyp (Scyphula of the Acraspeda, Coral polyp) arises out of a planula 

 in the following way (Fig. 99). The planula (A) attaches itself by the animal or 

 aboral end of its body {B). At the oral end the body of the ectoderm sinks in in the 

 shape of a pit and forms the ectodermal oesophagus with the external mouth {C). 

 The base of the cesophagus then breaks through in the direction of the gastral cavity 

 {D), and so arises the enteric aperture. The cesophagus of the Scyphula is at first 

 not a round but a flatly compressed tube {F). On each side of it a prolonga- 

 tion of the enteron penetrates between it and the ectodermal body wall— the first 

 2 gastric pouches. Crosswise to these there is a further growth of 2 new pouch- 

 shaped invaginations of the enteron between the ossophagus and body wall. Thus 

 arise around the oesophagus the 4 gastric pouches of the Scyphula ((?, mt). The 

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