6i6 AJOLLUSCA—APLACOPHORA. 



In some forms the animal is very long in the body and the shelly plates 

 are placed at intervals. The animals, which are vegetable feeders, may be 

 found clinging, lilce limpets, to the under surfaces of rocks and stones at low 

 tide, and when detached will coil up much after the fashion of the well-known 

 wood-louse. The muscular foot extends the whole length of the under 

 surface of the body ; the end of the snout is just visible ; there are no 

 tentacles. A row of small gills is seen along each sido of the body under the 

 edge of the mantle. A remarkable feature in the shell of many chitons calls 

 for speciil mention. The outer chitinoua layer of the valves is perforated 

 for the passage of sense organs, which, in the family Chitonidse, are in 

 part converted into eyes. 



The Polyplacophora are divided into families as follows, according to 

 details of sculpturing, etc., of the valves: — Lbpidopleoriu^, Ischno- 



CHITO:<ID^, MOPALHD^, ACANTHOCIIITIDiE, CkYPIOPIACIU-S:, ChIIONID^. 



Order II. — Aplacophora, or Solenogastra. 

 The Aplacophora, which are carnivorous, are so modified that they scarcely 

 resemble molluscs at all. The body is worm-like and there is no trace of 

 shell, but the much thickened outer skin contains 

 shelly spicules, which probably represent the spines 

 and scales of the Polyplacophora. The foot is 

 extremely reduced, or altogether wanting. The 

 gills are in a chamber at the latter end of the 

 body, into which chamber also the excretory 

 orifices open. Their blood is red. Two sub- 

 orders are distinguished : — 



Sub-order 1.- — Nbomeniina, in which the foot is 



reduced to a ridge sunk in a groove along the ventral 



^^- *• side of the body. 



(."chItodeema. The radula is of the usual many-toothed order, or 



wanting. There are four families : — Nbomen.id^, 



Prokeomeniidje, Donuersiid^, and Parameniid^. 



Sub-order 2. — Ch^todermatina, represented by a single genua Chmto- 

 derma, in which the body is extremely long and cylindrical ; the foot is 

 wanting, its position being merely indicated by a groove ; whilst the radula is 

 represented by a single tooth. 



CLASS II.— PELECYPODA, or LAMELLIBRANCHIATA. 



The principal characteristic of this class is the division of the shell into 

 two halves, the valves, which lie on either side, right and left, of the animal: 

 hence the class at one time received the name of Bivalvia. These valves are 

 usually connected by an elastic, chitinous ligament, and are further kept in 

 place, when the shell is closed, by a aeries of interlocking teeth that project 

 from their margins. The shell is closed by powerful adductor muscles, but 

 opens spontaneously, when the muscles relax, by the action of the ligament. 

 Each valve is a hollow, irregular cone, the apex of which, termed the beak 

 or umbo, is the point at which growth began, and is in fact formed by the 

 valve of the young shell. In very good specimens, when the urabones are 

 not worn or eroded, the two valves that were formed by the animal ere it 



