CNIDARIA 



61 



a cavity at the centre. This is the earliest trace of the 

 •pneumatophore, which consequently is formed as a solid 

 ingrowth of the ectoderm. After this the fundament of the 

 first larval tentacle becomes noticeable as a latei'al evagina- 

 tion of both body-layei's (Fig. 24 B, t). The fundament 

 of a second deciduous larval tentacle soon follows. The 

 bilaterally symmetrical structure of the larva is indicated 

 by the appearance of the tentacle, since that side of the 

 body to which the above-mentioned organ belongs corre- 

 sponds to the zone from which subsequently all the newly 



Fig. 24. — Two stages of development of Halistemma (Stephanomia) pictum (after 

 Metschnikoff, from Balfour's CompamKue Embryology). A, ciliated planula-stage ; 

 ep, fundament of the pneumatophore as an ectodermal ingrowth ; B, older stage 

 with central gastric cavity ; po, fundament of the first polypite ; f , fundament of 

 tentacle ; pp, pneumatocyst ; ep, its ectodermal envelope (pneumatosaccus) ; hy, 

 entoderm in the region of the pneumatophore. 



appearing buds gi'ow forth, the so-called ventral side of the 

 Siphonophore stock. ^ At the same time, by a transverse 

 constriction at the base of the tentacle, a separation into an 

 upper portion of the body, which becomes the stem and 

 pneumatophore, and a lower portion is indicated. The first 



* The designation of this as the ventral side can only be established 

 by comparison with other Siphonophore larvae. On the other hand, 

 Haeckel (No. 70, p. 315, Taf. xxii.) has pointed out that the primary 

 tentacle of similar larvae has a dorsal position. 



