754 
ZOOLOGICAL LITERATURE. 
appear at first sight to homologate with some of the lower forms of the 
Anrmlosaj the second, subsequeiitlj produced within or in close organic 
connexion with the first, is the true Echinoderm. The extreme form of this 
singular cycle, in which the development of a provisional zooid as a separate, 
independent, living organism is carried to its full extent, is by no means con- 
stant throughout the whole subkingdom, although its existence has been esta- 
blished for all the recent orders. In each order it appears to be exceptional j 
and in certain cases it is known to bo carried to ils most abnormal degree in 
one species, while in a closely allied species of the same genua the mode of 
reproduction differs but slightly from the ordinary invertebrate type. 
To avoid ambiguity in the discussion of such singular relations, I believe 
it is necessary to introduce certain new terms. For an organism which pos- 
sesses all the apparent characters of a distinct animal, which is developed 
from the germ-mass, and which maintains a separate existence before the 
appearance of the embryo, I would propose the teim pseuclemhryo j and for all 
the appendages which homologate with the whole or with parts of such a 
pseudembryo, even although they do not assume fully the characters of a 
distinct animal form, I would propose the term pseutlemhryonic appemluyes. 
The same prefix may distinguish the organs of the temporary zooid, where 
such exist — pseudostome, pseudocdey pseudoproct, &c. The reasons for the 
adoption of this series of terms, and the rejection, as applied to the provisional 
organisms, of the ordinary terms embryo ” and larva,” are discussed in a 
subsequent part of this memoir. 
A few hours after segmentation has been completed, the surface of tho 
mulberry mass still presents the mammillatic appearance due to tho persist- 
ence of the ultimate yelk- segment. This, however, gradually disappears, 
and the superficial layer becomes fused into a sheet of structureless sarcode. 
Observed during the process of development within the vitelline membrane, 
the pseudembiyo is at fii’st nearly regularly oval, with a uniformly ciliated 
surface. Usually, however, before the rupture of the vitelline sac, the ciliated 
bands characteristic of the free condition of the pseudembryo are evident, 
and a large depression indicates the position of the pseudostome, a smaller 
spot immediately behind the pseudostome affording the first trace of the 
pseudoproct. 
On escaping from the vitelline sac, the pseudembiyo is about 8 millims. 
in length, oval, slightly enlarged towards one extremity, and girded by four 
nearly equidistant transverse bands of long cilia. It consists throughout of 
structureless sarcode studded with oil-cells, endoplasts, and granules — semi- 
fluid towards the centre, where it is somewhat darker in colour and exhi- 
bits active molecular motion, and becoming more consistent towards the 
periphery, where it forms a firm smooth surface. 
At this stage the pseudembryo is irregularly oval and in form slightly 
contracted posteriorly, expanded and gibbous anteriorly, tlie anterior ex- 
tremity flattened or slightly cupped. The posterior extremity expands into 
a small rounded disk. Slightly compressed and examined by transmitted 
light, the pentacrinoid larva has but little altered from the description given 
above j the joints of the stem are somewhat lengthened, and the cup is rather 
more open by the growth and slight separation of the upper portions of the 
plates of the upper tier. The whole of the pentacrinoid is entirely invested by a 
thick layer of transparent sarcode, which is merely the substance of the body 
