136 ACRASPEDA. 



The primitive zooid of a Siplionophora stock is thus a Medusa. Like 

 Sarsia and Willsia this Medusa must be supposed to have been capable 

 of budding. The ordinary nectocalyces by their resemblance to the 

 umbrellas of typical Medusae are clearly such buds of the medusiform type. 

 The same may be said of the pneumatophore, which, as pointed out by 

 Metschnikoff, is identical in its development with a nectocalyx. Both are 

 formed by a 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 

 gastro vascular 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 hyclrophyllia 

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

 Stephanomia, Halistemma etc.) the hydrophyllium of the primitive 

 poly pi te (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 fret- 



swim ming. 



Acraspeda 1 - The embryonic development of several of the forms 

 of the Acraspeda has been investigated by Kowalevsky (No. 147) and 

 Glaus (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 embryonic 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 segmen- 

 tation cavity. The wall of the blastosphere next becomes invaginated, 

 giving rise to an archenteron (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 stomo- 

 doeum, which is placed in communication with the archenteron on 

 the absorption of the septum dividing them. The relation of the 

 stomodseum to the original blastopore has not been determined. 



At the point of attachment there is developed a peculiar pedal disc, 

 and around the mouth there appears a fold of epiblast which gives 

 rise to an oral disc (fig. 78 D). Two tentacles first make their ap- 



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

 , Khizostornidffl, and Lucernaridte. 



