CCELENTERATA. 165 



solid process of epiblast in which a cavity the cavity of the nectocalyx or 

 pneumatocyst is eventually hollowed out. Around this there appears a 

 double layer of hypoblast containing a prolongation of the gastrovascular 

 cavity ; and this is in its turn enclosed by a layer of epiblast which forms the 

 covering of the convex surface of the nectocalyx and the external epiblast of 

 the pneumatophore. 



The generative gonophores are clearly also zooids, and the hydrophyllia 

 are probably a rudimentary form of umbrella. In many cases (Epibulia, 

 Stephanomia, Halistemma etc.) the hydrophyllium of the primitive polypite 

 (manubrium) is absent. In such instances it is necessary to suppose that 

 the umbrella of the primitive zooid of the whole colony has become aborted. 

 Leuckart originally took a somewhat different view from the above in that 

 he regarded the starting-point of the Siphonophora to be a compound fixed 

 Hydrozoon stock, which became detached and free-swimming. 



Acraspeda'. The embryonic development of several of the 

 forms of the Acraspeda has been investigated by Kowalevsky 

 (No. 147) and Claus (No. 153). Their observations seem to 

 point to an invaginate gastrula being characteristic of this group. 



Amongst the forms with alternations of generations and a 

 fixed larval form Chrysaora and Cassiopea have been most fully 

 investigated. The ovum of the former undergoes the first embry- 

 onic phases while still in the ovary. In the latter it is enclosed 

 amongst the oral processes. A complete and more or less 

 regular segmentation leads to the formation of a single-walled 

 blastosphere with a small segmentation cavity. The wall of the 

 blastosphere next becomes invaginated, giving rise to an arch- 

 enteron (fig. 78 A). The blastopore soon closes up, and the 

 archenteron is converted into a closed sack completely isolated 

 from the epiblast (fig. 78 B). The surface of the larva becomes 

 in the meantime covered with cilia. The free larval stage thus 

 reached is similar to the ordinary Hydrozoon planula. After 

 the closure of the blastopore the larva becomes elongated, and 

 one end becomes narrowed. By this narrowed extremity the 

 larva soon attaches itself, and at the opposite and broader end a 

 fresh involution of the epiblast appears (fig. 78 C) ; this gives 

 rise to the stomodaeum, which is placed in communication with 

 the archenteron on the absorption of the septum dividing them. 

 The relation of the stomodaium to the original blastopore has 

 not been determined. 



1 I use this term for the group, often known as the Discophora, which includes the 

 Pelagida;, Rhizostomidas, and Lucernaridse. 



