MOLLUSCOIDA. 173 



writers) which leads by a short gullet into a capacious stom- 

 ach (d). From the stomach an intestine is continued to ter- 

 minate by a distinct anus, which does not communicate direct- 

 ly with the exterior, but opens into a second great chamber, 

 known as the "cloaca" or "atrium" (e). The cloaca, in turn, 

 opens on the exterior by the second, or atrial aperture, in the 

 test (b). 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 aper- 

 tures already spoken of as perforating the respiratory sac. 

 The cilia which fringe these apertures all work toward 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 Tunicata, but it has a very simple structure (1, h). 

 It consists of a simple tube, open at both ends, and not pro- 

 vided 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 contractions in one direc- 

 tion, and then propelled for a like period in an opposite direc- 

 tion; so that "the two ends of the heart are alternately 

 arterial and venous." 



The nervous system in the Tunicata consists of a single 

 ganglion placed on one side of the oral aperture. 



With one or two exceptions all the Tunicata are hermaph- 

 rodite, the organs of reproduction being situated in a fold 

 of the intestine, and opening into the cloaca. The embryo is 

 at first free, and in most swims about by means of a long tail, 

 so that it presents 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 

 organism consists of a number of zooids, produced by con- 

 tinuous budding, and connected together by a common tube, 

 through which the blood circulates. In the so-called " com- 

 pound" Ascidians the tests are fused together into a com- 

 mon gelatinous mass, in which the individuals are imbedded 

 in groups. Some of the Tunicata are oceanic that is to 

 say, are found floating, or swimming, at the surface of the 



