Characteristics of the Mollusca. Ixxxvii 



cTiaena, and Magiliis infest certain Coelenterata ; but, like certain 

 Polyzoa, such as Loxosoma and Pedicellina, which attach them- 

 selves to Vermes as to other marine olijects, are not parasitic in a 

 strict sense. 



The Sub-king-dom Mollusca contains two great divisions or pro- 

 vinces — the Mollusca proper, under which are comprehended as 

 classes the Cephalopoda, the Gasteropoda, the Pteropoda, and the 

 Lamellibranchiata ; and the Molluscoidea, under which are compre- 

 hended the Brachiopoda, the Tunicata, and the Polyzoa. The Mol- 

 lusca proper are distinguished firstly by the great development of 

 their org-ans of animal life. Their motor organs consist of a ' foot,' 

 which may be of very various shapes, and is divisible morphologically, 

 and sometimes actually, into a ' propodium,' ' mesopodium,' and ' me- 

 tapodium;' and of an ' epipodium,' developed by the foot proper along 

 its Hne of junction with the visceral mass. The names of the three 

 classes. Cephalopoda, Gasteropoda, and Pteropoda, relate to the dif- 

 ferences observable in these motor organs. The nervous system in 

 all four Classes of Mollusca consists of three pairs of ganglia at least, 

 which are sensory, parieto-splanchnic, and motor respectively ; and 

 which, being mutually connected by commissures, form a collar 

 round the commencement of the digestive tract. The organs of 

 vegetative life in the Mollusca contrast with those of the MoUus- 

 coidea in two chief points : firstly, their heart is all but invariably 

 provided with one or two auricles, in correlation with their more 

 perfectly developed and specialized respiratory apparatus, whence 

 they have been called 'Otocardia;' and secondly, their digestive 

 system is, also all but invariably, proctuchous. The three classes. 

 Cephalopoda, Gasteropoda, Pteropoda, are placed together in one 

 sub-division as ' odontophorous' Mollusca, in contradistinction to the 

 ' bivalve' Lamellibranchiata, by virtue of their uniformly possessing 

 the peculiar dentigerous rasping organ known as the tongue, and 

 of their never possessing a bivalved shell. With these differences 

 others are correlated, as will be detailed in the description of the 

 class Lamellibranchiata. 



The Molluscoidea, as a sub-division, are distinguished from the 

 Mollusca proper by the following characteristics. They are not 

 only, like the Lamellibranchiata, destitute of any prehensile or 

 masticatory apparatus, and dependent therefore upon ciliary action 

 for the ingestion of alimentary matters, but they are, with the 



