ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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upper and lower layer ; the former gives rise to the ectoderm, and the 
latter to the mesoendoderm. In the latter, before further differentiation, 
there appear three series of cavities — those of the coelom and of the noto- 
chord. 
The ectoderm gives rise to the nerve-ganglion, which is a thickening, 
and to the two peribranchial tubes, which are invaginations of the ecto- 
derm. These tubes become, in the course of development, separated 
from the ectoderm, grow forwards as well as backwards, and do not till 
later become connected with the independently formed cloacal orifice. 
Of the two mesodermal tubes, which are at first equally developed, the 
right alone continues to grow, and becomes the pericardial sac. The 
left tube breaks up into cells, which either remain separate or (possibly) 
take part in the formation of the cellular zone which surrounds the 
germinal disc. 
Sense-organ of Salpa.* — Mr. A. Bolles Lee gives an independent 
account of an organ imperfectly figured and described in Eussian by 
Ussow in 1876. In Salpa mucronata there are two of these organs; 
they are end-organs of a recurrent twig of the third nerve, and are 
symmetrically placed on either side. In a living specimen the organ 
may be seen to consist of a stem terminating in a bulb, which is sur- 
mounted by a delicate hyaline claviform appendage. The stem is a 
cellular tube formed by a process of the inner mantle. 
In good preparations the bulb may be seen to be composed of a 
central tuft of sense-cells and a surrounding calyx of supporting cells ; 
the latter varies a good deal in form. The minute details of structure 
are described, and the author sees much that is plausible in the view 
that the organ is either a taste-bulb or was one once. But on the other 
hand, a little reflection shows that while the Salpa has in its cellulose 
mantle a highly watery and highly hygrometric jelly, it has in this 
organ one whose shape must be affected by change in the density of the 
circumambient water ; these changes would pull on or relax the sensory 
hairs of the organ, and it, probably, is a hydxometric apparatus. 
/3. Bryozoa- 
Cristatella.t — Mr. C. B. Davenport has investigated the origin and 
development of the individual in this colonial Bryozoon. He finds that 
most individuals give rise to two buds, one of which forms a new 
branch, while the other continues the ancestral branch. The median 
buds migrate to a considerable distance from the parent polypide before 
giving rise to new buds. Descendants from common ancestors, equal in 
age, are arranged similarly in the same region of the colony. New 
branches are formed on either side of ancestral branches. 
The greater the difference in age between the youngest and the next 
older bud, the greater the distance between the points at which they 
begin to develope. In typical “ double buds ” both polypides arise from 
a common mass of cells at the same time. From the neck of old 
polypides a stolon-like process of cells is given off to form median buds. 
The alimentary tract is formed by two e vagina tions of the bud, and 
* Quart. Journ. Micr. Sci., xxxii. (1891) pp. 89-97 (1 pi.), 
f Bull. Mus. Comp. Zool., xx. (1890) pp. 101-52 (10 pis.). 
