CCELENTERATA. 155 



the aboral side, which is considerably more prominent than the oral one. 

 They contain a hypoblastic axis. The aboral end continues to grow and 

 the tentacles gradually assume a horizontal position. A constriction now 

 appears, dividing the larva into an aboral portion which will eventually form 

 the stalk, and an oral portion. At the apex of the latter a row of short 

 tentacles the future oral tentacles now appears. The larva has at this 

 stage the form known as Actinula. In this condition it becomes hatched, 

 and shortly afterwards it becomes fixed by the aboral end and grows into 

 a colony. 



The development of Myriothela (Allman, No. 150) takes place on the 

 Tubularian type. The ovum invested by a delicate capsule becomes freed 

 by the rupture of the gonophore, and is then taken up by the remarkable 

 claspers characteristic of the genus. In the claspers it becomes fecundated 

 and undergoes its further development. After segmentation a gastric 

 cavity is formed, and provisional tentacles arise as a series of conical 

 involutions which subsequently become evoluted. Permanent tentacles are 

 formed as conical papillae on a truncated oral process. After hatching it 

 has a few days' free existence, and then becomes attached, and loses its 

 provisional tentacles. 



Although Hydra itself constitutes the simplest type of Hydrozoon, its 

 development, which has been fully investigated by Kleinenberg (No. 161), is 

 in some respects a little exceptional. The segmentation is regular, but a 

 segmentation cavity is not formed. The peripheral layer of cells gradually 

 becomes converted into a chitinous membrane, which is perhaps homologous 

 with the perisarc of marine forms. Between the membrane and the germ a 

 second pellicle makes its appearance. The above changes require about 

 four days for their completion, but there next sets in a period of relative 

 quiescence which lasts for some 6 8 weeks. During this period the 

 remaining development is completed. The cells of the germ first fuse 

 together. In the interior of the protoplasm a clear excentric space arises, 

 which gradually extends itself and forms the rudiment of the gastric cavity. 

 The outer shell in the meantime becomes less firm, and is finally burst and 

 thrown off, owing to the expansion of the embryo within. 



The outermost layer of the protoplasm becomes, relatively to the inner 

 layer, clear and transparent, and there thus arises an indication of a division 

 of the walls of the archenteric cavity into two zones, or layers. These layers, 

 which form the epiblast and hypoblast, are definitely established on the 

 appearance of cells with contractile tails 1 in the clear outer zone, between 

 which the interstitial epiblast cells subsequently arise. 



The embryo, still forming a closed double-walled sack, elongates itself, 

 and at one pole its wall becomes very thin. And at this point a rupture 

 takes place which gives rise to the mouth. Simultaneously with the mouth 

 the tentacles become formed as hollow processes, according to Mereschkowsky 

 two being formed first and subsequently the others in pairs. Very shortly 



1 These cells are the so-called nerve-muscle cells. Their nature is discussed in 

 the second part of this work. 



