52 SHELLS AND SHELL-FISH. PART I. 



when viewed in reference to any doubts which may 

 hang over this group, from its isolated situation on one 

 side of its own circle. The strong analogy between the 

 Parenchymata and the annulose Vermes has induced 

 all writers to blend them together, even up to this 

 day ; the one being the most imperfect of all the Tes- 

 tacea, as the other is of the Annulosa. Passing on to 

 the relationship between the cuttlefish (Cephalopoda) 

 and the barnacles {Cirripeda), we are struck with a 

 resemblance much stronger than would be supposed 

 to result from a mere relation of analogy. In both 

 these tribes, the mouth is surrounded by long slender 

 arms, employed to catch their prey ; both contain ani- 

 mals whose softer parts are protected by shells. Never- 

 theless the Cirripedes have no more to do with Mollusca, 

 than the latter have with those crabs, which, like the 

 genus Cyclops, are naturally enveloped in bivalve shells. 

 To the Radiata, again, the analogy of these tribes is no 

 less apparent ; for, as both have the organs of motion 

 arranged around, or rather radiating from, their mouth, 

 which thus becomes the common centre, they have, to 

 all outward appearance, the essential characters of ra- 

 diated mollusks. 



(42.) Assembling all the groups thus brought into 

 comparison in the following table, we shall see, at one 

 glance, the mutual relations they present in their com- 

 ponent parts. 



Analogies of the Test aceovs Mollusca to the Annulosa. 



r Typical, the most highly organised;'^ 

 Gasteropoda. -J head distinct, with long antenna or >Ptilota. 



C tentacula. 3 



rHead indistinct, confounded with thei 

 DiTHYRA. ^ thorax, or altogether wanting; no>-ApTEEA. 



C antenna; or tentacula. J 



VT ...„,. f Disk of the belly flattened, and often 7 •.,„^, 



Nudibranchia. J performing the oIHce of a foot. j Annelides. 



TThe most simj^le in their organisation, ^ 

 Parenchvmata. -^ naked, and crawl upon tlieir belly j >V£ 

 C no percej)tible branciiia. J 



r Mouth surrounded by long tentacula, T 

 ■< or arms ; soit parts of the body ge- >( 

 C iierally protected by a shell. j 



ERMES. 



