STRUCTURE OF 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 powertof forming a shell, includes two classes, of 
which 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- 
Fig. 63.—Soctan AscrpIans. 
panying figure (fig. 63). The cavity of the mantle possesses, 
as in the former instance, two orifices ; by one of which, }, 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, ¢, and the intestinal canal, 7, which terminates 
near the aperture for the exit of the water. All these parts 
