PENNATULA PHOSPHOREA. 37 



still more dorsally situated, space being provided for these new 

 ones by the lateral growth of the rachis itself. This view also explains 

 the fact, noticed above, that the most dorsal polype of each leaf 

 projects across the middle line over to the opposite side, this being the 

 only direction in which its growth is not opposed by neighbouring 

 polypes. (2) That the uppermost leaves are the first-formed ones, 

 and therefore the oldest, and that new leaves are formed at the 

 bottom of the rachis below the previously-formed ones, the lowest 

 leaf being always the youngest. 



These conclusions agree completely with what we have stated 

 already concerning Fuiiieulina, in which, as in Pennatnla, developinent 

 of the polypes appears to proceed from tlie dorsal towards the ventral 

 surface, and from below upwards, the ventral polypes being always the 

 largest and oldest, and the dorsal ones the smallest and youngest. 



The zooids, or rudimentary asexual individuals, cover as already 

 noticed the whole ventral surface of the rachis excepting the median 

 groove, svhich is often barely perceptible in the upper half of the Pen. 

 They also extend up the sides of the rachis between the bases of the 

 leaves (Fig. 3 e), and form on the dorsal surface little clusters between 

 each pair of leaves. The zooids exactly reverse the arrangement we 

 have found to hold among the polypes, the ventral zooids being the 

 smallest and the dorsal clusters invariably the largest. In the case 

 of the younger leaves these dorsal zooids are not much smaller than 

 the youngest or most dorsal polypes, and it is possible that they may 

 develope into them, as we have siipposed to occur in Fitniculina. We 

 have not, however, had siifficient material at our disposal to enable ua 

 to determine this point. 



i.— Anatomy of the Polypes. — 



The polj'pes of PennatuJn agree iu all essential features with those 

 of Funiculina already described,* the differences, which are of merely 

 secondary importance, being due mainly to the fusion of the polypes 

 to form leaves in Pennatnla. 



The structure of the polypes is shown in Figs. 3, 4, and 5 ; Fig. 3 

 representing a whole leaf, with its component polypes ; Fig. 4, a 

 longitudinal section through one polype taken along the line AA in 

 Fig. 3 vertically to the surface of the leaf ; whilst Fig. o represents a 

 transverse section of the leaf along the line BB iu Fig. 3, the section 

 cutting the six most ventrally-situated polypes of the leaf at different 

 points of their length. 



We propose now to consider the several parts of the polype, 

 taking them in the same order as in the description of FuniculitKi. 



a. The Body-wall consists, as in Funiculina, of a firm gelatinous 

 mesoderm (Fig. 5 .r), clothed on its outer and inner surfaces by 

 ectoderm (iv) and endoderm (;/) respectively. The mesoderm, and 

 therefore the body-wall of which it forms the greater part of the 



* Supra, pp. 25—36 



