Morse. ] 320 (March 19, 
to apply the term laterality as distinguishing the Mollusks, while 
the term /ergality was applied to the whole Cuvierian branch of 
Articulates. Agassiz has also called attention to the fact that while 
the display of structure is upon the sides in Mollusca, it is upon the | 
back in the Articulata, though numerous and important exceptions 
occur in both groups. | 
In the worm, the locomotor muscles are intimately connected | 
through the entire length of the body with the integumentary system, 
especially on its dorsal and lateral walls (Rolleston). In the Mol- 
lusk, on the contrary, the locomotor muscles are connected ventrally __ 
with a specialized creeping disk, the foot. | 
In the Mollusk, with few exceptions, the viscera are carried above 
the foot in a protruding chamber. 
Fig. 3. 
Molluscan archetype from Carus. 
In the worm, the symmetry of the body is never disturbed by 
the viscera. The tegumentary envelope, when separate from the 
body proper, forms at most a projecting, or an everted collar about 
the head, and, in a few instances, a continuous free membrane along 
the sides of the body. In the worm this envelope, and adjoining 
parts possess chitinous outgrowths in the shape of scales, rarely a 
shell, but commonly sete, the latter being a marked character of 
the Vermes. 
In the Mollusk the tegumentary envelope is prolonged, and often- 
times continuous, forming a sack or mantle, inclosing a conspicuous 
cavity, and protecting the gills. Hence the name Saccata, proposed 
by Prof. Hyatt. Setze, or scales, are not present; while the possession - 
of a calcareous shell, composed of one or more pieces, furnishes the 
only material to nine-tenths of those who study them. In the worm> 
the plates, when present, and the thickened integument, are per- 
forated with minute tubules, a character not possessed by the Mollusk. 
In the worm, the digestive canal is straight, rarely convoluted, 
and suspended freely in the perivisceral cavity. (See Fig. 1, 6.) 
