STRUCTURE OP TUNICATA. 121 



of the nutritive apparatus) than the corresponding parts in 

 Articulated animals, in which the apparatus for locomotion so 

 much predominates. 



114. The group" of Acephalous Mollusks which are desti- 

 tute of the power*of forming a shell, includes two classes, of 

 Avhich one does not depart widely from the general Molluscan 

 type, whilst the other presents 'so strong a general resem- 

 blance to Zoophytes, that until recently it has been universally 

 ranked with it. The first of these classes receives its name 

 TUNICATA from the circumstance that the mantle, instead of 

 secreting a shell, is very commonly condensed into a tough 

 leathery or cartilaginous tunic. Many of these animals live 

 separately, and have the power of freely moving through the 

 water. Others are associated in compound masses, of which, 

 however, the individuals are not connected by any internal 

 union. But others form really composite structures, like 

 those of Zoophytes ( 124) ; each individual being able to 

 live by itself alone, but being connected by a stem and vessels 

 with the rest. The general structure of the individuals is 

 the same, however, in the single and in the composite 

 animals of this class, and may be understood from the accom- 



t 



Fig. 63.--SOCIAL ASCIDIANS. 



panying figure (fig. 63). The cavity of the mantle possesses, 

 as in the former instance, two orifices ; by one of which, 5, a 

 current of water is continually entering, whilst by the other, 

 a, it is as continually flowing out. These orifices lead into a 

 large chamber, the lining of which, folded in various ways, 

 constitutes the gills ; and at the bottom of this chamber lie 

 the stomach, e, and the intestinal canal, i, which terminates 

 near the aperture for the exit of the water. All these parts 



