No. 3.] MORPHOLOGY OF THE ACTIATOZOA. 31 1 



to the Sterrula, and there are physical difficulties in the way of 

 immigration. 



There has been described, however, among the Alcyonarians 

 a single case of invagination following the formation of an 

 archiblastula. Haeckel ('75) states that he has found such an 

 occurrence in Monoxenia. I have not been able to consult his 

 original account ('75 a) of this phenomenon, and cannot accord- 

 ingly determine the value of the statement ; but in view of what 

 is known to occur in other Alcyonaria, and since he also states 

 that he has found the same thing in an Actinian, — which, as I 

 shall later endeavor to show, seems doubtful, — I think it is not 

 safe to rely on the accuracy of the statement until it is con- 

 firmed by further observations. 



Up to the present, we know nothing as to the formation of 

 the germ-layers in the Edwardsias, the recent representatives 

 of the ancestors of the Hexactiniae ; but the Alcyonarians must 

 be regarded as standing with the Edwardsiae at the bottom of 

 the Actinozoan stem, since the two groups agree in the number 

 of their mesenteries, — a feature of fundamental importance in 

 the present state of our ideas on the subject of the evolution of 

 the Actinozoa, the difference in the mode of arrangement of the 

 longitudinal muscles being of secondary value. The occurrence 

 of a Sterrula, and its formation by delamination in this some- 

 what primitive group of Actinozoa, is accordingly of importance 

 as affording a basis whereon to build an explanation of what is 

 found in the Hexactinioe. 



The most definite statements made hitherto as to the early 

 development of the Hexactiniae are by H. V. Wilson ('88), for 

 Manicma. Segmentation results in the formation of a hollow 

 blastula, and by the delamination of the blastula cells, the blas- 

 tocoel becomes filled up with a mass of yolk-bearing cells whose 

 outlines and nuclei cannot readily be made out. The embryo is 

 then a Sterrula, and agrees in the manner of its development 

 with the Alcyonaria. Jourdan's description ('80) of the devel- 

 opment of Balanophyllia does not extend to stages sufficiently 

 early to enable one to state what the method of formation of 

 the germ-layers is ; but it is probable that it is closely similar 

 to what occurs in Manicina, and a Sterrula (see his Fig. 126, 

 PI. XVII) is the result. 



In A. parasitica {Adamsia Rondeletii of Andres), Kowalew- 



