Ixxxviii Introduction. 



exception of the Nectascidiae and a few Polyzoa^ also devoid of organs 

 for motion from place to place^ at least in their adult state. The 

 entire sub-division is aquatic, and, with the exception of a part of 

 the Polyzoa, is marine. Most of its members are monoecious, and 

 many are social, which the Mollusca proper never are. They 

 always have a more or less indurated external envelope, which in 

 two of the classes, the Tunicata and the Polyzoa, into which the 

 MoHuscoidea are divided, is sacciform ; and in the third, the Bra- 

 chiopoda, takes the form of a bivalve shell. In this latter case the 

 nerve-system attains a higher development in certain species than 

 it ever does in either of the other two classes of Molluscoidea ; but 

 as no Molluscoid has a ' foot,^ pedal ganglia are never developed, 

 nor the three pairs of ganglia characteristic of the higher sub- 

 division of the sub-kinsrdom attained to. 



Class, Cephalopoda. 



Mollusca, in which the foot proper has its margins split up into 

 tentacles, or into acetabuliferous arms, which are arranged so as to 

 form a corona round the mouth. The epipodia, which in Pteropoda 

 are the principal, remain, in Cephalopoda, important locomotor 

 organs, forming as they do, by their partial or perfect coalescence, 

 the ' fimnel,^ which is lodged in their capacious neurally-situated 

 mantle cavity, and which by the contraction of the muscular walls 

 of that cavity has water so projected into it as to effect the peculiar 

 backward swimming movement characteristic of the class. Move- 

 ment from place to place in the way of crawling is effected by the 

 midtifid foot proper. By virtue of the high evolution of their 

 organs generally, and especially of those of animal life, such as the 

 eyes, the Cephalopoda are l^y common consent placed at the head 

 of the Molluscan Sub-kingdom; by the retention, however, of a 

 bilateral arrangement relatively to a median antero-posterior plane 

 in many organs, and especially in those of vegetable life, they show 

 indications of aflSnity to lower Mollusca, which are lost in the inter- 

 mediate classes of Gasteropoda and Pteropoda. The Cephalopoda are 

 divided into two orders, according to the number of theu' gills ; the 

 TetrabraucJiiata being the less, and the Bibranchiata the more 

 highly organized of the two. All Cephalopoda possess an internal 

 cartilaginous framework which supports and protects their nerve- 



