Respiration in Invertebrate Animals. 295 



The external machinery of the process of breathings however 

 unlike the constituent parts, or different the principle of its 

 action, does not therefore appear to involve any difference in the 

 ultimate products of the nutritive actions of the organism. 



The suppression of motive cilia in the Arthropoda is the signal 

 of the saving of power. The economized force is diverted to 

 other purposes. The nervous and vascular centres are raised in 

 standard : the whole muscle-system is augmentedly developed, 

 and the secernent organs are woven into more complex structures. 

 The presence of chitine in the dermal skeleton of the Articulata 

 entails a distinctive character upon the periphery of the circu- 

 latory system. Contractile vessels cannot exist in the substance 

 of an incontractile solid. This segment of the circulation of 

 Insects should be studied with special reference to this point. 

 When the skeleton is very thick, it is composed of a series of 

 superimposed laminae, between which are tunnelled certain chan- 

 nels, as in bone, for the conveyance of the nutritive fluid. In 

 the centre of the larger of these channels tracheae may be de- 

 monstrated*. 



The true epidermis of Insects is always and universally com- 

 posed of a tessellated hexagonally-celled epithelium. The ana- 

 tomical characters of the ultimate blood-channels of Insects will 

 be most successfully studied in the corresponding parts of the 

 circulatory apparatus of the Crustacea. The same description 

 essentially applies to both. 



Crustacea. 



Every Crustacean is a water-breathing, every Insect an air- 

 breathing animal. To this rule there can be found no real, 

 many apparent, exceptions. In the system of the Crustacean 

 there exist no water trachese. Although the Crustacean is an 

 insect breathing water, the mechanism contrived to accomplish 

 the process is comparable in no single particular with that 

 used in the instance of the Insect breathing air. In the former 

 plan there is no wonder-striking singularity. The apparatus 

 employed is common to every aquatic animal. The organs of 

 breathing in every true crustacean conform essentially to the 

 aquatic type. Though some species seem to enjoy the power of 

 respiring on the atmospheric plan, the apparatus used fulfils the 

 requirements of the branchial principle. 



One typal form of blood-corpuscle prevails throughout all the 

 species of this class. The fluid, in the embryonic state, as in 

 the larvae of Insects, and antecedently to the evolution of the 

 branchiae and the heart, presents a description of corpuscle dif- 

 ferent from that which afterwards in the same animal charac- 



* See Histological Catalogue by Prof. Quekett. 



