CLASS I. AMPHINEURA, von Ihering. 



(= Isoplenra, Ray Lankester'; Aculifera, Hatschek.) 



THE Amphiueura are a group of Mollusca characterized, firstly, by 

 their more or less elongated and quite symmetrical body, with the 

 mouth and the anus situated at its two ends ; and, secondly, by their 

 mantle, which is always provided with numerous spicules imbedded 

 in a cuticle. The nervous system consists of 2 lateral and 2 ventral 

 parallel cords, meeting in a cerebral ganglion. Head without tentacles 

 or eyes. 



All the Amphineura are marine in habit. They are found in all 

 oceans and at nearly all depths. They existed in very ancient geo- 

 logical ages, for they are already present in the Lower Silurian. 



There are two very distinct orders (1) Polyplacophora ; (2) Apla- 

 cophora. None of the latter have been recorded from New Zealand. 



ORDER 1 POLYPLACOPHORA, de Blainville. 



Dorsal surface bearing 8 imbricating shelly plates ; head divided 

 from the body ; gills numerous, occupying a lateral groove on each 

 side between the foot and the encircling mantle ; foot adapted to creep- 

 ing. They are unisexual, the genital organs and nephridia paired. 

 The radula is well developed. 



Vernacular Name. Mail-shell. 



THE SHELL OF THE POLYPLACOPHORA. 



In the description of the Chitons technical terms are unavoidable, 

 and the following definitions are copied from Pilsbry's excellent mono- 

 graph : 



The shell in Chitons consists of 8 imbricating pieces or valves, bound 

 together by a leathery girdle of connective tissue. The valves, when 

 freed of the girdle by soaking a few hours in water, are seen to be of 

 three forms : the anterior (or " head ") valve, semicircular in out- 

 line, its apex elevated ; the intermediate (sometimes called " central ' : 

 or " median") valves, squarish in shape; and the posterior (or "tail") 

 valve, which is like the intermediate valves, with the addition of a 

 sloping surface behind the apex or mucro. In structure, the valves are 

 composed of two layers, generally quite different in colour and texture an 

 outer layer, called by Middendorf the tegmentum ; and an inner, the 

 articulamentum. 



The surface of the valves (tegmentum) is divided in nearly all Chitons 

 into clearly defined or indistinct areas. The intermediate valves (I) 

 are divided into lateral areas and a central area ; the latter being 

 subdivided into a dorsal or jugal tract, extending t long the ridge of 

 the valve, and two pleura or pleural tracts, occupying the side slopes 

 in front of the diagonal line or rib. In some forms (II) the diagonal 

 line is obliterated, the lateral areas and the pleural tracts being united 

 into a single uniformly sculptured expanse, the latero-pleural area, on 

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