350 Proceedings of the Royal Physical Society. 
covered by a thick layer of coUetoderm, which is con- 
tinued over the body of the polyp, and which, when the 
polyp retires within its tube, fills up the top of the 
tube by its cushiony folds, so that the polyp is completely 
hidden, and the funnel appears as it were closed by a valve. 
The colletoderm in my specimen was coated and impreg- 
nated with mud. Mr Alder's specimen w^as covered with 
grains of fine sand. I was at first inclined to believe that 
this zoophyte was merely a variety of Atractylis repens, 
w^hich, with its medusoids, I have already described to the 
Society ; but after it had been in captivity a few days, I 
found that it was beginning to put forth ovisacs, one on 
opposite sides of the polyp-stems (Plate XV. fig. 7). 
The mode of reproduction in this zoophyte is unique 
amongst the Tubulariadse, though I have noticed and de- 
scribed it in the Sertularias and Campanularias. 
The female generative sac oi Atractylis arenosa resembles 
that of Hydractinia ; it is a simple sac formed of ectoderm , 
or the outer layer of the coenosarc, enclosing a similar sac 
of endoderm, the " placenta," the whole being covered by a 
layer of scleroderm and colletoderm. Between the placenta 
and the ectoderm a large number of ova are developed, each 
showing a germinal vesicle and spot (fig. 8). When the ova 
are sufficiently advanced for extrusion from^the generative 
cavity, the investments of the sac are ruptured, the sac 
assumes a long, cylindrical form (fig. 9), and a most labo- 
rious process of parturition commences. With each pain 
the ectoderm of the sac contracts laterally, like the bell of 
a Medusa, and at the same time the placenta (fig. 9 c) is 
dilated by fluid pumped into it from the somatic cavity of 
the zoophyte, so that the ova, which are floating in a milky 
fluid, are forced against the summit of the generative sac. 
Meanwhile, another process has been going on, — the ex- 
ternal surface of the summit of the sac has been secreting a 
thick cap of gelatinous colletoderm (fig. 9 d), which is to 
form a nidus for the further development of the ova. The 
contractions become still more violent, until the ova are 
confined in a mass at the dilated upper part of the sac ; this 
last is ruptured, and they are forced into the gelatinous cap, 
