VOL. uu.) Balanogtossus. 197 
The mesoblastic pouches have arisen as two paired evaginations 
from the lateral walls of the digestive tract. These four pouches be- 
come entirely severed from their original connection, and form large 
thin walled, entirely closed bags. They become so large in fact that 
each pair almost entirely surrounds the digestive tube, their inner 
walls being in contact with this latter while their outer walls are in 
contact with the inner surface of the ectoderm. In short, they form 
the real body cavity, or ccelom. The heart is a peculiar structure. 
It is said to arise as a space, merely, between the water vesicle and 
the proboscis gland. The walls of these two latter organs become 
_. closely pressed against each other, the contact being interrupted in 
a small area only, and this is the heart which becomes filled with a 
fluid in which there are no cellular elements. This makes the walls 
of the heart to consist of parts of the walls of two other organs, 
and this means that if each of these has a function of its 
own the tissue of the heart has a triple office, viz.: the portion 
forming a part of the wall of the water vesicle functions in that 
capacity; the portion belonging to the proboscis gland performs its 
office there, and finally the two parts together perform the functions 
of a heart. The organ can be very distinctly seen in the living larva 
when placed on its side and flattened down somewhat with a com- 
pressor (Fig. 4 is drawn from such a preparation). The walls are 
very distinct, and the contractions constant and regular. It should 
be pointed out, however, that the contractions are of quite a differ- 
ent character from what is seen in the hearts of most other animals. 
It does not consist either in a uniform, simultaneous contraction of 
the entire wall, as one sees take place, for instance, in the spherical — 
vascular organs on the sides of certain marine leeches; or of a wave 
of contraction passing from one end to the other, the contraction 
affecting the entire circumference at each successive point passed 
over by the wave, as takes place in peristaltic movement, 
or as is seen in the heart of Ascidians, for example. But one- | 
half of the wall does not contract at all, while in the other half 
a sharp fold sink deep into the cavity of the organ and travels 
across it, the edge of the fold not extending across, however, to 
the opposite wall. 
The gills have arisen as paired pouches from the dorsal wall of 
the cesophagus, the-anterior pair appearing first and the others in 
succession behind them. They do not fuse with the ectoderm and 
