6 



SHELLS AND SIIELL-FISH. PART I. 



Testacea. This tribe is the Cephalopoda, or cuttlefiph. 

 It is among these only^ out of the whole class of Mol- 

 lusca, that we find a distinct, and often rigid, back-bone, 

 — not, indeed, jointed or articulated, but performing the 

 same office as the vertebrated column of a quadruped 

 or a bird : others of its class have no limbs whatever ; 

 but the cuttlefish have immense arms, which are also 

 used as legs ; by these they crawl with the greatest 

 facility on the bottom of the sea, swim quickly in the 

 water, and retain the most forcible hold of such ani- 

 mals as they prey upon. The eyes of nearly all Mol- 

 lusca are either small and indistinct, or are entirely 

 wanting ; in these, however, the eye is large, and as 

 fully developed as in any vertebrated animal. There 

 cannot, therefore, be the least doubt of the close ap- 

 proximation of the Cephalopoda to the Vertehrata ; and 

 as a large proportion of the fossil species were fur- 

 nished with spiral shells, it follows that the testaceous 

 class comes immediately after the Vertehrata. This 

 affinity being established, we are at no loss to discover 

 the point of junction between this and the next class, 

 or the Jcrita. The singular naked animals forming the 

 Nudihranchia of Cuvier, have been very properly 

 placed by many authors close to the tunicated polypes 

 (^Tunicata). Again, on the other side of the circle of 

 Acrita, we have the genus Zoanthus and the Poli/pes 

 charnues of the French Avriters, closely connected to the 

 Radiata by Friapulis and other kindred forms ; while 

 the last vestige of the radiated structure is seen in the 

 Cirripeda : these latter animals, however, by their arti- 

 culated limbs, are yet removed out of the confines of 

 the Radiata, although they become the first group in 

 the Annulosa, — thus standing in precisely the same re- 

 lation as the Cephalopoda do to the Vertehrata. ^Ve 

 have now traced the chain of affinity into the annulose, 

 or insect class ; and it only remains to inquire how 

 these latter are connected to the Vertehrata, or, in other 

 words, how the animal circle is made out and closed. 

 On this point, again, the labours of our predecessors 



