138 ALCYONID.E. 



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

 from the strobilization of the fixed Scyphistoma form of other Acraspeda. 



Alcyonidse. In the Alcyonidse 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 invagination. 



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 jUcyoniwrn palmatum the impregnation is external. The seg- 

 mentation 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 Clavularia crassa (No. 168) there is a complete 

 segmentation followed by a delamination. The larva of Al. 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 larvae 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 stoniodaeuni. 

 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 ovuni in the previous stage. It is to be 

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

 of the stomodseal invagination the gastric chamber is placed in free 

 communication with the spaces between the mesenteries 1 . During the next 

 stage the young Alcyonium also acquires eight tentacles, which arise as 

 hollow papillae opening into the eight mesenteric chambers. By this stage 

 also the matter filling up the mesenteric chambers is nearly absorbed. 



Between the epiblast and hypoblast there is formed an homogeneous 

 membrane, which penetrates in between the two layers of hypoblast 

 which form the mesenteries. On the outer side of this membrane, and 

 therefore presumably derived from the epiblast, is a layer of connective- 

 tissue cells, which eventually gives rise to the abundant gelatinous tissue 

 (crenenchyrna) in which the skeletal elements are deposited. In Sympodium 

 coralloides Kowalevsky (No. 168) has shewn still more completely the deriva- 

 tion of the stellate mesoblast cells from the epiblast. He finds that the 

 calcareous spicula develope in these cells as in the mesoblast cells of 

 sponges. The branched gastro- vascular canals in this tissue are out- 

 growths of the primitive enteric cavity. A layer of circular muscles is 

 formed at a late period from the epiblast, but the longitudinal muscles of 



1 The German abstract is very obscure as to the formation of the mouth. 



