310 



Introduction. - 



-MOI,LUSCA.■ 



-Af.p.angement. 



other cenlies, connected with the lower side of the 

 esophageal ring, send nerves to the foot, viscera, and 

 respiratory organ." — ( Woodward.) 



ARRANGEMENT OF MOLLUSCA. 



Previous to the time of Cuvier, the animals which 

 we now know under the name of ]\Iolhisca, were con- 

 fusedly arranged, and very imperfectly known. They 

 " were intermingled with worms and with zoophytes, 

 while a great number of them stood detached from 

 their allies under the ordinal designation Testacea, 

 merely because they were inclosed in hard calcareous 

 shells ; the knowledge of the inferior tribes being then 

 too little advanced to admit of the application of any 

 characters but those that were derived from exterior 

 form anil con.sistence. By liis mimerous careful dissec- 

 tions, Cuvier was early enabled to detect and appre- 

 ciate the unnatnralness of the prevalent systems ; and 

 when liis labours had convinced him that their over- 

 throw was necessary to the progress of science, they 

 had at the same time furnished him with the materials 

 out of which he sought to erect a new system, which 

 has been of incalculable advantage to scientific concho- 

 logy, and which remains untouched in all its giand 

 lineaments, though his successors have certainly im- 

 proved and worked out many of the minor details." — 

 (Johnsiiin.) It is to Cuvier, then, that we owe the 

 grouping together the animals to which the name of 

 MoUusca is now properly restricted. 



This great naturalist took as the first division the 

 possession or the want of a head, and arranged the 

 various classes into two principal groups: — 1st, Those 

 which had a bead more or less distinctly formed, the 

 Encejihala ; and 2nd, Those which were destitute of 

 that important appendage, the Accphala. In the first 

 group he placed the Cephalopoda, or Cuttle-fishes ; the 

 Ptcropoda, Winged or Floating mollusca ; and the 

 Gasteropoda, or Snails. In the second gronp he placed 

 the ConcIi/'/era, or Bivalve shells; the Brachiopoda, 

 Brachiopods, or Lamp shells ; and the Cirrliopoda, or 

 Barnacle shells. This last class is now ascertained to 

 belong to the Crustacea, but may be replaced by the 

 Tunicata, or Timicaries — which he made merely an 

 order of the Acepihala. Various arrangements have 

 lieen suggested by naturalists since Cuvier put forth 

 his system, but we will only mention one, that of Dr. 

 Gray ; merely premising that however varied the 

 methods propounded have been, they all hinge in 

 reality upon that of the celebrated author of the " Eegne 

 Animal." 



Cuvier divided the Mollusca into two great groups, 

 according as they possessed or as they wanted a head. 



Dr. Gray, in adopting the classes of Cuvier, divides 

 them into two great groups also, but takes as the basis 

 of his arrangement the possession or want of a foot. As 

 this arrangement is the one adopted in our great national 

 collection in the British Museum, and has the merit of 

 being patent to all who wish to study Conchology ; and 

 as we shall have frequent occasion to refer to it in the 

 following pages, we subjoin it as follows : — 



SUB-KINGDOM MOLLUSCA. 



CiRourl. — Pedifera, crawling on a foot placed under the 

 body. 



Clasal. — G ASTEEOFODA— Gasteropods, Umvalves,orSiwils. 

 Head distinct, with eyes and tentacles ; body usually 

 protected by a conical, more or less spiral shell, often 

 I'liruished with an operculum. 



Class 2. — CONCHIKERA — Co«c7i{/t'rs or Bivalves. 



Head indistinct ; mouth placed between the gills ; they 

 and the body inclosed between the two leaves of tlin 

 mantle, which are covered by two shelly valves united 

 above by a ligament. 



Group II. — Atoua. Foot none, or very rudimentary. 



Chiss 3. — Brachiopoda — Brachiopods or Lamp Shells. 

 Mouth placed at the base of two Bpirally-tvri^trd 

 ciliated arms between the two leaves of the mantle, 

 which are covered with two separate shelly valves ; they 

 live attached to other marine bodies. 



Class 4. — PTERoroDA — Pteropods or Floating Mollusca. 

 Uead prominent, with one or rarely two pairs of tins 

 on the side of the neck, by means of which they suim 

 on the ocean ; body often covered with a thin conoidal 

 shell. 



Class 5. — CErilALOPODA — Cephalopoils or Cuttle-fishes. 

 Head distinct, large, with eight or ten or more arms 

 by which they walli bead downwards. 



To these we must add — 



Class 6. — TuKiCATA — Tunicated Mollusca or Tunicaries. 

 No distinct head; mouth at the bottom of the bran- 

 chial sac ; body protected by an elastic gelatinous tunir, 

 with two orifices, one for the admission of water and 

 food into the interior, and the oth.er for the cxpuLiuu 

 of excrementitious matters ; no shell. 



As in the preceding parts of this work, the plan 

 followed has been, in general, to commence with the 

 most highly-organized forms, descending to those lower 

 in the scale, we will deviate here from the sketch just 

 given of the arrangement in the British Museum, and 

 begin with the Cephalopods. " The type of structure 

 which characterizes the Dibranchiate order of Cepha- 

 lopods," says Professor Owen, " places these mollusca 

 not only at the head of that division of the animal 

 kingdom, but in respect to its closer proximity to the 

 vertebrate type, unquestionably at the head of the 

 whole invertebrate series " Premising tliis, we will 

 therefore commence with — 



Class I.— CEPHALOPODA— Head-walking Molluscs. 



TllF, Cephalopods are the most highly organized of all 

 the molluscs. They are symmetrical animals, having 

 their right and left sides equally developed. They 

 have a large distinct head, a month armed with power- 



ful horny jaws, acting vertically like the mandibles of 

 birds ; a large and fleshy tongue, and large well-devel- 

 oped eyes, approaching in structure those of verte- 

 hrated animals. The body, wliich is soft and fleshy, is 



