MOLLUSCOIDA. 189 
small non-retractile tentacles, and opens into a great chamber 
known by various names, but best as the “respiratory sac.” 
This sac occupies the greater part of the cavity of the body 
(fig. 97, 1 c), and has its walls perforated by numerous apertures, 
the sides of which are ciliated. At the bottom of the respira- 
tory sac is a second opening (the south of some writers) which 
leads by a short gullet into a capacious stomach (d). From the 
stomach an intestine is continued to terminate by a distinct 
anus, which does not communicate directly with the exterior, 
but opens into a second great chamber, known as the “cloaca” 
or “atrium” (¢). The cloaca, in turn, opens on the exterior by 
the second or atrial aperture in the test (4), These two great 
chambers—namely, the respiratory sac and the cloaca—occupy 
the greater part of the body-cavity, and where their walls come 
into contact, a free communication is established between the 
two by means of the ciliated apertures already spoken of as 
perforating the respiratory sac. The cilia which fringe these 
apertures all work towards the cloaca, and thus a constant 
current of water is caused to set in by the oral aperture, through 
the respiratory sac, into the cloaca, and out again by the atrial 
aperture. In this way respiration is effected, the walls of the 
respiratory sac being almost made up of blood-vessels. A distinct 
heart is present in all the Zzuzcafa, but it has a very simple 
structure (1 #). It consists of a simple tube, open at both 
ends, and not provided with valves, In consequence of this, 
the circulation in the majority of Tunicaries is periodically | 
reversed, the blood being driven for a certain number of con- 
tractions in one direction, and then propelled for a like period 
in an opposite direction ; so that “the two ends of the heart are 
alternately arterial and venous.” 
The nervous system in the Zwszcata consists of a single 
ganglion placed on one side of the oral aperture. 
With one or two exceptions all the Zumzcata are hermaphro- 
dite, the organs of reproduction being situated in a fold of the 
intestine, and opening into the cloaca. The embryo is at first 
free, and swims about by means of a long tail, so that it pre- 
sents considerable resemblance to the tadpole of a frog. 
The Tunicata are all marine, but differ a good deal from one 
another in form, In the so-called “simple” Ascidians the 
animal has the shape figured above, and is fixed to some solid 
object by one end of the test. In the “social” Ascidians the 
