6 CHARACTERS OF THE MOLLUSCA. 



the appearance of a mantle or cloak, a name borrowed by 

 naturalists to designate this part. "When the animal is naked, 

 the cloak is thick and mucous ; but when covered with a shell, 

 as it mostly is, the transparency and tenuity of the covered 

 parts prove how much it has felt the influence of a protecting 

 medium. 



But this character of indivisibleness of the body and defi- 

 ciency of members will not distinguish the Mollusca from all 

 avertebrate animals ; for many of the Radiata, and all the 

 Zoophytes are in the same condition. From the former, how- 

 ever, they may be known by the disposition of their organs, 

 which do not tend to or radiate from a centre, by the mucous 

 nature of their skin, for in the Radiata it is either a calcareous 

 or leathery coat, or a thin pellucid pellicle, and their locomo- 

 tive organs are in the form of tubular processes or papilla?, 

 or long pendant membranes ; while the Mollusca move on 

 a flattened disk, or by a tongue-like solid foot, or swim by 

 means of certain folds of their mantle. They are with greater 

 difficulty to be distinguished from Polypes. With most 

 of them indeed there is no uncertainty : their superior size, 

 the existence of a shell, their peculiar form and locomotive 

 powers are all obvious distinctive characters ; but when you 

 come to investigate the subject with more minute care, you 

 will find it difficult to tell a compound tunicated Mollusk 

 from one of the more highly organized Polypes : and modern 

 discovery seems likely to prove that the lines of demarcation 

 are ideal. As it is, I know of no distinctions but which are 

 anatomical. All Mollusca have a heart and blood-vessels, all 

 have traces of a nervous system, and all have an internal 

 respiratory apparatus ; but Polypes have no heart and no 

 vessels, no nerves nor ganglions, and the respiration is exter- 

 nal, — the tentacula which encircle the outer margin of the 

 mouth being the organs of that primary function. 



The Mollusca form a subkingdom, the natives of which 

 may be divided into two great sections : those which have a 

 head, more or less distinctly formed, we name the cephalous, 

 and the acephalous are of course those which are deprived of 

 this part, and in which the mouth lies hidden in the bottom 

 of the mantle. 



Among the former, the Cephalopods occupy the first rank. 

 They derive their name from the circumstance of their feet 

 being placed upon the head, and in a circle round the mouth, 

 a character which suffices of itself to distinguish them from 

 all others. They are represented by the Cuttle-fish ; and it 

 is presumed that all shells formed like the shell of the Nau- 

 tilus, are tenanted by animals of similar conformation. Their 



