a2 ZOOLOGY. 
88, close the aperture by an operculum attached to the posterior and upper - 
part of the foot. It is sometimes smaller than the aperture, and can be 
drawn in some distance; in other cases it fits the aperture exactly. In 
texture it is either thin and horny, or thick and shelly, sometimes increasing 
obliquely or spirally, by deposition upon one side, and at other times 
enlarging concentrically, but always taking the shape of the aperture. In 
the genus Hipponyx the anomaly is presented of the operculum being 
attached to the rocks upon which the animal is found. In dry seasons the 
land-snails protect themselves by a temporary operculum, formed by a 
slimy secretion, which hardens, and thus closes the aperture. 
Most bivalves, as the name implies, are composed of two valves 
(pl. 76, figs. 82-34), united by a ligament upon the back; but some. of these ~ 
have accessory pieces (fig. 49), which are not of sufficient importance to 
remove them from their class. Even the anomalous form, Aspergillum 
(pl. 75, fig. 71), belongs to the Bivalves or Conchifera; for, although it is a 
shelly tube, pierced at the anterior extremity like a pepper-box, an 
examination will disclose a small open bivalve shell, solidly imbedded in the 
shelly material of the tube. This is roughly represented near the upper end 
of the figure, although the artist probably did not recognise its true character. 
The nervous system of the Malacozoa is not symmetrical, as in the 
Articulata, nor radiated, as in the Radiata, but the ganglia are distributed 
unsymmetrically from the brain, or chief ganglion, situated above the 
cesophagus. : 
The Malacozoa are divisible into three sub-divisions or classes, the lowest 
containing the Acephala (pl. 76, fig. 34, &c.), named Acephalophora by 
Blainville; the next the Gastropoda (fig. 1, &c.), named Paracephalophora 
by Blainville, and including the Pteropoda, according to this author; and 
the highest the Cephalopoda (pl. 76, figs. 16, 17, 75-77). 
Class 1. Acephala. 
This class contains the four orders Bryozoa, Tunicata, Brachiopoda, and 
Conchifera. The sections, here named orders, are by some naturalists 
considered to be classes, which they sub-divide into orders of a different 
value. Indeed, it is extremely difficult to construct groups which shall have 
the same value under the same name, in different departments of Zoology ; 
and there is no special rule which can be followed in all cases. 
Milne Edwards, observing that the Bryozoa, which had previously been 
confounded with the Zoophyta, bear certain near relations to the Tunicata, 
united the two in a sub-division of the true mollusea, and Cuvier had 
previously shown that the Tunicata are related to the Conchifera. 
Dujardin gives to the group of Bryozoa the same value among the Mollusca 
that he assigns to the Conchifera and Brachiopoda. Agassiz also unites 
them to the Acephala, of which he considers them to be the lowest order, 
corresponding to the Foraminifera, to which he assigns the lowest place 
among the Gastropoda. In fact, the Bryozoa have much resemblance to 
the Mollusca in their alimentary canal, which is quite different from that of 
the Zoophyta. 
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