4 CRUSTACEA. 



of this branch of the Articulata, and not so of any true Mollusca. 

 Again, the organs of the mouth as well as the jointed structure of the 

 legs are completely Crustacean in type. 



The Rotifers have the mandibles and other mouth-organs of Crus- 

 tacea, and some of them resemble certain Entomostraca in general 

 form, and in the jointed structure of the caudal extremity. In these 

 species we have, therefore, the lowest Crustacean form under a 

 Eadiate type, the type of the inferior branch of the animal kingdom. 



Crustacea also pass, by almost imperceptible shades, into Vermes, 

 through the Caligus and Lernsea tribes; the most degraded Lernaaan 

 forms having the sluggishness and almost memberless character of 

 the lowest worms. Their resemblance to the typical Crustacea is so 

 slight, that, without a knowledge of the gradations through the well- 

 modelled Caligi to the higher forms, their relations to the class would 

 hardly be suspected. 



2. Characteristics. On account of the wide variations among Crus- 

 tacea, the systematist experiences great difficulty in laying down the 

 characteristics of the class. 



The higher divisions have a regular heart ; the lower (and this is 

 true even of some Caligidse) have no heart, and only two or three 

 valves in the course of the circulation. 



The higher have a system of vessels for the arterial circulation, the 

 venous system only being lacunal; the lower have no vessels for 

 circulation in any part, and the blood sweeps along among the muscles 

 in broad currents, flowing off in side channels wherever passages are 

 open, more like the sap in the leaf of a plant than what is naturally 

 looked for in the zoological kingdom. 



The higher orders have branchiae for the aeration of the blood 

 attached to the thoracic members ; species of another type have allied 

 organs attached to the abdominal members; and those^of the lower 

 orders are without any trace of branchiae or corresponding organs, and 

 the function of aeration devolves upon the exterior surface of the 

 body. 



The higher orders have a nervous ganglion for each segment of the 

 body, and these ganglia are clustered in two masses only when the 

 limbs are gathered closely about a centre, with the abdomen small 

 and inflexed against the thorax, as in Crabs ; but in the lower orders, 

 although the body may consist of a series of segments, we find at 

 times only one single ganglion, pierced by the oesophagus, and placed 



