COAT-OF-MAIL SHELLS. 615 



The early stages in the development of the young Mollusca are the same as 

 in all other animals. The original single cell splits up into many cells, till it 

 looks like a mulberry. These cells next arrange themselves in a single layer, 

 so as to form a hollow ball ; one side then becomes folded in, so that a 

 double layer of cells results, enclosing a cavity which ultimately becomes the 

 digestive tube ; other cells arise between the two layers, that subsequently 

 form the various organs and the muscles of the young animal. Meantime 

 there arises a swimming organ, which is peculiar to the young mollusc ; this 

 is called the velum. It begins as a ridge, fringed with fine hairs (cilia), 

 encircling that part of the head of the embryo which lies in front of the 

 mouth, and it finally expands into a sort of disc, which is drawn out into 

 lobes or into fingers. This is known as the veliger stage, and is further 

 marked by the formation of the young shell and the first appearance of the 

 foot. The young shell is always a simple coil, even in those forms which, 

 when adult, have a straight shell, or discard their shell altogether. As the 

 young snail grows up the velum disappears. 



The molluscan sub-kingdom is divided into five classes : 



I. AMPHINEURA. 



II. PELECYPODA, Or LAMELLIBRANCHIATA, 



III. SCAPHOPODA. 



IV. GASTROPODA. 

 V. CEPHALOPODA. 



Formerly the sea-butterflies PTEROPODA were considered to form a class by 

 themselves : they are now, however, known to be only very greatly modified 

 forms of Gastropoda. 



CLASS I. AMPHINEURA. 



The Amphineura are in many respects the most primitive of all the Mollusca. 

 They are literally symmetrical ; that is to say, if the body were divided down 

 the middle from head to tail, structures found on one side of the body would 

 be seen to be repeated on the other. 



The elementary canal is not bent, the hinder end opening at the opposite 

 extremity of the elongate body to the mouth. 

 The nervous system consists of two lateral and 

 two ventral nerve trunks, which start from 

 the cerebral ganglia and are connected together 

 at intervals by numerous transverse nerve cords. 

 The class is divided into two orders : POLYPLA- 

 COPHORA and APLACOPHORA. 



K 



ORDER I. POLYPLACOPHORA. 

 The Coat-of-Mail Shells, or Chitons (Fig. 2), are 

 readily distinguished by being alone amongst the 

 Mollusca in possessing a shelly covering of many 

 pieces called the lorica. There are eight plates 

 or valves in all on the back of the animal, fitting 

 one over the other like the tiles on a roof. 

 Generally the lorica is surrounded by a leathery 

 girdle, so called, which helps to unite the several 

 plates, and which, though sometimes naked, is Fig. B. 



usually studded with scales or beset with spines. A CHITON* 



