No. 401.] DEVELOPMENT OF PENNARIA TIARELLA. 401 
first it is an exceedingly delicate, almost indistinguishable, 
transparent film about the base of the polyp. This gradually 
thickens and soon hardens into a sheath about the growing 
polyp, covering at first the entire larva. Annulation of the 
perisarc seems to occur at no very definite time or place in its 
growth. In some’ cases it is apparent almost from the first. 
In others it only becomes apparent at a considerably later 
time, and nearer the hydranth than the base. As to the sig- 
nificance of the annulations, either in origin or function, nothing 
very definite can be said. That the perisarc itself is a protect- 
ive adaptation seems almost beyond question; but whether 
the annulations characteristic of this and many other hydroids 
is an additional adaptation, rendering the stem flexible, etc., 
may be doubtful. 
VII. ABNORMALITIES. 
The eccentric forms of cleavage already considered naturally 
raise the question as to probabilities of corresponding anoma- 
lies among the larvæ of these forms, and also of the resulting 
polyps; whether at any rate any variation appears from the 
normal type of embryo. The answer in part may be inferred 
from a glance at Pl. I, Figs. 4-6. These represent but three 
out of a considerable number of eccentric forms observed dur- 
ing the progress of the work. As will be seen, Fig. 4 repre- 
sents what might be designated as a twin planula, having a 
rather broad and blunt anterior and a bifurcated posterior. 
In Fig. 5 is represented a second type, quite common, which 
differs from the former chiefly in the shape of the body of the 
embryo, which is somewhat spindle-shaped, and also in the 
slender and attenuated character of the posterior bifurcated 
portions. In Fig. 8 is shown a third type which is some- 
what unusual. The body portion is decidedly eccentric in 
shape, with irregular tentaculate processes arising from it at 
various points and of various sizes and shapes. 
Whether such anomalies occur to any extent under perfectly 
natural conditions, of course, cannot be said. But from their 
occurrence under the most favorable conditions in aquaria, and 
Since, moreover, they seemed in no way to interfere with subse- 
