CGELENTERATA. 167 



Aurelia is stated by Kowalevsky to develop in the same way as Cassiopea; 

 and the one stage of Rhizostoma observed is that in which it has a (probably 

 invaginate) gastrula form. 



In Pelagia the ovum directly gives rise to a form like the parent. The 

 segmentation and the invagination take place nearly as in Cassiopea, but the 

 archenteric cavity is relatively much smaller, and the large space between it 

 and the epiblast becomes filled with the gelatinous tissue which forms the 

 umbrella. The blastopore does not appear to close but to become directly 

 converted into the mouth. As in Cassiopea the larva takes a somewhat 

 four-sided pyramidal form. The mouth is placed at the base. The pyramid 

 becomes subsequently flatter, and at the four corners four tentacles grow out 

 which increase to eight by division. The flattening continues till the larva 

 reaches a form hardly to be distinguished from the Ephyra resulting from 

 the strobilization of the fixed Scyphistoma form of other Acraspeda. 



AlcyonidaB. In the Alcyonidae the segmentation appears 

 always to lead to the formation of a solid morula, which becomes 

 a planula by delamination. The true enteric cavity is formed 

 by an absorption of the central cells, but the axial portion of the 

 gastric cavity and mouth are formed by an epiblastic invagi- 

 nation. 



The development of these types has been mainly studied by Kowalevsky 

 (147), and my knowledge of his results is derived from German abstracts of 

 the original Russian memoirs. 



In Alcyonium palmatum the impregnation is external. The segmenta- 

 tion is very exceptional in character. It commences with the formation of a 

 series of irregular prominences on the surface of the ovum, which become 

 segmented off to form a superficial layer of epiblast cells. The inner mass 

 of protoplasm then divides up into polygonal cells to form the hypoblast, 

 which would thus seem to be formed by a kind of delamination. In Clavu- 

 laria crassa (No. 168) there is a complete segmentation followed by a 

 delamination. The larva of A I. palmatum elongates and becomes ciliated, 

 and so assumes the characters of a typical planula. The central hypoblast 

 is formed of an outer granular stratum with imperfectly differentiated cells 

 the true hypoblast and an inner homogeneous mass with vacuoles. 



Some of the larva? become fixed, while others coalesce together and 

 form a large mass, the fate of which has not been further studied. An 

 invagination of epiblast takes place at the free end of the fixed larva, which 

 gives rise to the so-called gastric cavity, i.e. the axial portion of the general 

 enteric cavity, which would appear to be in reality a kind of stomodaeum. 

 Around the gastric cavity the hypoblast forms eight mesenteries, the 

 chambers between which are filled with the homogeneous material which 

 occupied the centre of the ovum in the previous stage. It is to be 

 presumed, though not stated, that by an absorption of the blind end 

 of the stomodasal invagination the gastric chamber is placed in free 



