LECTURES ON MOLLUSCA. 155 



finished) is for the most part a reproduction of Adams' Genera with 

 the addition of fossils ; and is chiefly valuable for its copious and accu- 

 rate figures of shells illustrating the subgenera. The following pages 

 are intended simply as an introduction to any of the above works. 

 Books of older date are necessarily so full of errors that they should 

 not be studied till after the student has become familiar with, the present 

 means of knowledge. 



Shell-making animals have been so little known, that we have no 

 English word to express them. They are commonly called ' l 'shell- 

 fish," because most of them live in the sea. "Fish" are, properly 

 speaking, cold-blooded vertebrates breathing by gills. It is a strange 

 assemblage which groups with these the warm-blooded whales ; the 

 oysters and whelks ; the jointed craw-fish ; and the radiated star-fish. 

 Just as we have been obliged to import the Latin word mammal, to 

 include men, whales, bats and tigers, which are all warm-blooded, and 

 suckle their young ; so we must import the word molluslc, to include 

 snails and slugs, oysters and clams, cuttles and tunicaries ; all of which 

 agree in having soft bodies without jointed limbs ; the nervous system 

 being irregularly distributed in knots, or ganglia, the principal of 

 which surrounds the throat like a collar. 



In general shape, they are very dissimilar from each other. Some 

 have a large head with staring eyes ; others are blind and headless. 

 Some have many feet, others one, while whole classes have no organ 

 of locomotion whatever. Some are so highly organized that many true 

 fishes have to confess their inferiority : while some have special organs 

 so little developed that it is doubtful whether they should be called 

 degraded mollusks or superior zoophytes. 



It is by no means a necessary condition of a mollusk to be shell- 

 bearing. The lowest tribes have none ; in the highest they are only 

 occasional or rudimentary, or are altogether absent ; the land and sea 

 slugs are destitute of hard parts ; and some even of the bivalves are 

 almost entirely horny. The name "shell-fish" therefore, as applied 

 to the whole group, will have to be given up ; because myriads of species 

 live on land and breathe air, and even the water species are not true 

 fish ; and because a large proportion of them have no shells. 



Mollusks form one of the five great primary divisions of the Animal 

 Kingdom. They rank side by side with the Articulata, or Jointed 

 Animals, which include Spiders, Insects, Crabs, Worms, &c. The 

 Sea- Worms, which have calcareous shells ; and the Barnacles which 

 formed part of the " multivalve shells" of Linnreus, but which are now 

 known to be degraded crabs, used to be considered mollusks, and are 

 still seen in collections of shells.* Strange as it may seem, these 

 apathetic creatures have much closer relationship with spiders and 

 butterflies. The mollusks are specially designed for eating ; the artic- 



* The Cirripedes were thought by early naturalists to be the fry of Barnacle Geese. Very 

 learned descriptions are on record, illustrated by figures accurately representing the author s 

 imaginations, showing how the barnacles grew upon trees in the water, and at last came forth 

 from their shelly eggs as full-flown birds. The reality is scarcely less surprising than the 

 story: for it is now known that these creatures begin life as an active little crab, with legs, 

 head and eyes all complete, swimming about in the open sea. Instead of developing how- 



