52 ZOOLOGY. 



8S, close the apertui'e by an (yperculwm 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 apertm-e exactly. In 

 texture it is either thin and horny, or thick and shelly, sometimes increasing 

 obliquely or spirally, by deposition ujjon 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 

 {pi. IQ^figs. 32-34), 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 

 {pi. 7 5, fig. 71), belongs to the Bivalves or Conchifera ; for, although it i8 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 

 oesophagus. 



The Malacozoa are divisible into three sub-divisions or classes, the lowest 

 containing the Acephala {pi. 76, fig. 34, »kc), named Acephalophora by 

 Blainville ; the next the Gastropoda {fig. 1, etc.), named Paracephalophora 

 by Blainville, and including the Pteropoda, according to this author ; and 

 the highest the Cephalopoda {pi. 76, figs. 16, 17, 75-77). 



Class 1. Acephala. 



This class contains the four ordere 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 mollusca, 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. 

 256 



