402 
ORIGINAL ARTICLES. 
is resolved into an oval ciliated animalcule, composed throughout, 
and consisting entirely, of homogenous structureless sarcode. This 
sarcode germ increases rapidly in size, at first by the absorption of 
organic pabulum through the entire surface, as in the simple forms 
of Protozoa. Special locomotive ciliated fringes are developed, at 
first simple in their arrangement, but afterwards becoming extremely 
complicated by the unequal development of the various parts of the 
organism. As Mr. Huxley has shown in his important analysis of 
Professor Muller’s researches, # these fringes have essentially the 
same disposition in all Echinoderm pseudembryonic forms. They 
are transverse to the axis of the body, one girding the body before 
the mouth, the other behind the mouth and before the anus. In 
some forms the number of these bands is increased, blit in all their 
arrangement is the same. A special absorbent and assimilative tract 
is now hollowed out in the sarcode substance, a large buccal aperture 
with a well marked muscular oesophagus, a stomach, a short curved 
intestine, and an anal pore, 
16. Throughout the greater part of the body and the swimming 
appendages, the sarcode seems to be consistent and continuous, but 
in one part of the body (§ 8 and 12), usually in front of the mouth, 
and extending backwards on one or both sides of the stomach, there 
is a special cavity lined by a consistent membrane, or by a firmer 
layer of the sarcode substance, and ciliated. 
This cavity contains a special fluid in which definitely formed 
organic corpuscles are suspended. I suppose these ciliated cavities 
or coeca must be regarded as representing the vascular system of the 
pseudembryo, but they are afterwards brought into such immediate 
and important connection with the embryo that we may rather con¬ 
sider them with special reference only to its development. It is of 
great importance, however, in determining homologies in different 
pseudembryonic forms, to remember that the bipinnario consists of 
two essentially distinct parts, first, of a distorted cylinder of irritable 
sarcode provided with a locomotive apparatus, and absorbing nourish¬ 
ment through its whole surface, and by a special region dif¬ 
ferentiated for the purpose; and, secondly, of a sac or cavity contain¬ 
ing a corpusculated fluid elaborated by the sarcode zooid. This sac 
never actually becomes part of the Echinoderm embryo, but it is 
immediately in connection with its ambulacral vascular system, and 
the corpusculated fluid contained in it passes into the embryo and 
circulates in its vascular system. The cavity is then essentially an 
appendage of the Echinoderm ambulacral system. 
17. When these organs and parts are fully formed the develop¬ 
ment of the bipinnaria as a distinct organism stops. It presents, 
however, all the essential characters of an independent animal, and 
may maintain an independent existence and fulfil all the functions of 
organic life for weeks or months without further change. It has a 
* Annals and Magazine of Natural History, July, 1851. 
