CLASSIFICATION OF THE INVERTEBRATA. 153 



some advance in complexity of structure, others positively retrograde. A great number of kinds of 

 several groups of the Invertebrata live the lives of parasites 011 or within other animals, and the 

 great differences in shape and gifts which are noticed in the life cycle of these creatures are in relation 

 to this fact. Some positively exist on the juices of their unwilling host, others are so placed that they 

 help themselves to the food of their fellow-feeder, or to the supply set apart for its offspring. 



The great division of the Animal Kingdom whose members have these characters cannot be 

 classified as simply as the Vertebrata. The range of structural peculiarities is vast from the 

 shapsless microscopic entity, which can only be separated arbitrarily from the lowest and simplest 

 plant to the great Cuttle-fish. It does not appear possible to arrange the groups of the division 

 in an ascending series by the nature of their nervoxis and other structural developments or intelli- 

 gence, but several are on a level, as it were. Again, the history of the past does not assist us 

 in explaining the succession of the Invertebrata on the globe, for in the oldest rocks, which afford 

 evidence of a satisfactory nature, the highest amongst the Invertebrata are represented. 



The classification, which is as natural as is possible under the circumstances of existing 

 knowledge, but which, nevertheless, is very artificial, is as follows : 



The Invertebi'ata are divided into great types, or groups. 



1. T/te Mollusca. These are animals with a soft body, without segments, naked or covered with 

 a shell of one or two valves composed of carbonate of lime, secreted by a fold of the skin -the 

 mantle. They have a brain mass, and foot and mantle ganglia. Some have an internal hard 

 shell, or cartilage. The symmetry of the body is bilateral. Example, Cuttle-fish. 



2. The Arthropoda. The body is in ringed segments of various shapes, provided with limbs ; 

 the brain is united to a ganglioriic cord, which passes along the ventral surface within. The 

 symmetry is bilateral. Example, the Common Fly. 



3. Tlw Vermes. The body is either without segments, or may be composed of nearly similar 

 segments, without articulated limbs. The symmetry is bilateral. Example, the Earth-worm. 



4. The Echinodermata. The body, or part of it, is arranged in a radial manner, the 

 divisions being generally five in number. The skin contains or is covered by a symmetrical 

 armour of plates of carbonate of lime. The digestive and circulatory organs are distinct and 

 separate. There is a nervous system, and locomotion proceeds by ambulant tentacles. Example, 

 the Sea Urchin. 



5. The Zoophyta. The body is arranged in a radial manner in divisions of four or six, 

 or their multiples. A visceral cavity serves for digestion and circulation. Examples, the Coral 

 and the Jelly-fish. 



6. Protozoa. Minute animals, with very simple organisation ; structures, slightly differentiated ; 

 often unicellular. Examples, Animalcules and Sponges. 



These great divisions are not exactly defined in nature, and they are subdivided into secondary 

 groups, and are also united in some instances by forms of life which cannot well be placed in 

 any particular one. 



INTERMEDIATE GEOUPS. 



1. The Tunicata have a more or less leathery or cartilaginous covering sac, which is more 

 or less tub-shaped ; a gullet with perforations leading to a respiratory cavity, surrounded by 

 an inner skin, which envelops the viscera also. There is a simple nervous ganglion placed dorsally, 

 and a rudimentary heart. The symmetry is to a certain extent bilateral. They may be placed in 

 the neighbourhood of the Vermes and Mollusca in their classification. Example, the Ascidian. 



2. The Mollmcoida have the body with shells placed differently to those of the Mollusca, 

 or have a tubular or shell-like covering. The gills are more or less free and fringed with cilia, 

 without the usual lamellae of the Mollusca, and they serve the process of the capture of food as well 

 as of respiration, or there may be a crown of ciliated tentacles. The Bryozoa and the Lampshells, 

 or Brachiopoda, are included in this group, and in their structures, embryonic and adult, they 

 show resemblance to those of Vermes, Mollusca, and Tunicata. 



These types or great groups are subdivided into classes, orders, families, genera, and species, 

 which will be indicated in the description of their natural history. EDITOR. 

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