CHAP. II. PRIMARY DIVISIONS OF TESTACEA. 35 



posed of the DITHYRA of Aristotle,, or the bivalves, 

 whose structure is less perfect, but which are in like 

 manner protected by a regularly formed, and often 

 richly coloured, bivalve shell. The third, or aberrant 

 group, as usual, comprehends three : 1 . The NUDIBRAN- 

 CHIA of M. Cuvier, or the naked Gasteropoda; 2. The 

 PA RENCHYM ATA, or intestinal Testacea; and, 3. The CE- 

 PHALOPODA, or cuttlefish. We shall first slightly glance 

 at the leading characters of each of these classes, and 

 then endeavour to trace their analogies to other animals. 

 (30.) Under the name of GASTEROPODA, we retain 

 the greater number of Cuvier's divisions*; but the most 

 typical consists of the univalve or spiral Testacea, whose 

 body is protected and generally coveredt by a calcareous 

 shell : the head, although not assuming a very decided 

 shape, is always present, and is, in almost all instances, 

 at once distinguished by having two or more tentacula, 

 assuming the appearance of the antennae and the palpi 

 of insects. All the true Gasteropoda possess the rudi- 

 ments of eyes ; in the generality, these organs appear 

 in the form of black specks, either at the tips of the 

 longest pair of tentacula, as in the common snail, or at 

 their base. We thus find that the true Gasteropoda 

 possess four primary characters, the power of crawling 

 upon their belly, the possession of a head and eyes, 

 and, finally, the protection of a shell. Those which 

 are most remote from the point of perfection, and 

 which form the links of connection to other classes (as 

 the Tectibranchia on one side, and the Nudibranchia 

 on the other), have some one, or, perhaps, two, of these 

 characters either lost, or but very imperfectly developed; 

 these, in short, are the aberrant groups, and must never 

 be looked to for the right understanding of the class to 

 which they belong. The slug, for instance, has no 

 shell, or merely the rudiments of one ; yet it crawls 

 upon its belly, it is furnished with a distinct head and 

 even jaws, it has four tentacula, and a pair of rudi- 



* As the Heteropoda, Cyclobranchia, Scutibranchia, and Pvlmonaria. 

 f Except in Limax, &c. 



D 2 



