66 



EMBRYOLOGY 



/"._J \ 



three smaller ones arranged symmetrically were to be noticed. The 

 Ratariffi are characterized by the possession of a sail which stands ver- 

 tically on the upper surface of the elliptical disc, and the base of which 

 originally occupies the direction of the long axis of the disc, so that in 

 general the Ratariae possess a biradial structure. It is only in subse- 

 quent stages that the amphitectal (klinoradial) fundamental form of the 

 Vellelidse is brought out by the sail turning at an angle of forty-five 

 degrees to the above-mentioned axis, so that it stands diagonally. 



We have still to add something 

 on the laws of growth of Sipho- 

 nophore stocks. In those forms 

 which are characterized by an 

 elongated stem, the different indi- 

 viduals do not bud on the entire 

 circumference, but only along a 

 longitudinal line (Fig. 28). Since 

 the wall of the stem exhibits a 

 different structure along this line, 

 a cross-section of the stem pre- 

 sents a bilaterally symmetrical 

 arrangement. That side of the 

 stem from which the individuals 

 bud is known as the ventral side 

 (Glaus). The fact that the indi- 

 / y^'\/ '\ ^ viduals of the stem appear to be 



'* oriented in various directions re- 



sults from a spiral twisting of the 

 stem, by which, for example, the 

 biserial or multiserial arrange- 

 ment of the nectocalyces on the 

 nectosome is brought about. It 

 was shown by Clads (No. 62) 

 that in the Physophoridae the 

 spiral twisting of the nectosome takes place in the opposite 

 direction to that of the lower portion of the stem. 



As appears from Fig. 28, a budding point for the indi- 

 viduals of the nectosome is found at the upper end of the 

 stem. Another budding point, at the base of the necto- 

 some, supplies in general the buds for the rows of individuals 

 of the stem. Accordingly those groups of individuals which 



r ^^ ^ 



Fig. 28. — Young Agalmopsis 

 (after Gkgenbaur). a, stem; b, 

 pneumatophore ; c, only polypite 

 developed ; d, buds of tentacles 

 and dactylozroida belonging to 

 the group of individuals of the 

 first polypite ; e, hydrophillium ; 

 g, buds (nectocalyces) of the nec- 

 tosome ; h, buds of the lower 

 portion of the stem. 



