BODY-CAVITY i i 



{e.g. Mysis) the first maxillipede is a typical biramous limb, 

 though the expanded gnathobases in some forms are beginning 

 to project (Fig. 1, E), while the limb following, which corresponds 

 to the second maxillipede of Decapods, is simply a biramous 

 swimming leg. Besides this obvious conversion of a biramous 

 into a foliaceous limb, fui'ther evidence of the fundamental 

 character of the biramous type is found, first, in its invariable 

 occurrence in the Nauplius stage, which does not necessarily 

 mean that the ancestors of the Crustacea possessed this type 

 of limb in the adult, but which does imply that this type of 

 limb was possessed at some period of life by the common 

 ancestral Crustacean ; and, second, the limbs of the Trilobita, 

 a group which probably stands near the origin of the Crustacea, 

 have been shown by Beecher to conform to the biramous 

 type (Fig. I, H). Furthermore, the thoracic limbs of Nebalia, 

 an animal which combines many of the characteristics of. 

 Entomostraca and Malacostraca, and is therefore considered as 

 a primitive type, despite their flattened character, are really built 

 upon a biramous plan (Fig. 1, G). 



In conclusion, we may point out that this view of the 

 Crustacean limb, as essentially a biramous structure, agrees with 

 the conclusion derived from our consideration of the segmenta- 

 tion of the body, and points less to the Branchiopoda as 

 primitive Crustacea and more to some generalised Malacostracan 

 type. 



So far we have shortly dealt with those systems of organs 

 which are clearly affected by the metameric segmentation of the 

 body ; we must now expose the condition of the body-cavity to 

 a similar scrutiny. If we remove the external integument of a 

 Crustacean, we find that the internal organs do not lie in a 

 spacious and discrete body-cavity, as is the case in the Annelids 

 anc^A^ertebrates, but that they are packed together in an irregular 

 system of spaces (" haemocoel ") in communication with the 

 vascular system and containing blood. In the Entomostraca and 

 smaller forms generally, a definite vascular system hardly exists, 

 though a central heart and artery may serve to propel the blood 

 through the irregular lacunae of the body-cavity ; but in the 

 larger Malacostraca a complicated system of arteries may be 

 present which pour the blood into fairly definitely arranged 

 spaces surrounding the chief organs. These spaces return the 



