60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



When more than seven segments appear to be visible externally, as 

 is the case in the syncarid Anaspides, the additional supposed seg- 

 ments are due to slight creases or furrows in the body wall and are 

 not true segments with their necessary complement of muscle. Some 

 shrimps also apparently have many additional segments in the distal 

 part of the legs, but neither are these true segments, as their myology 

 proves. 



The so-called exopodite of the trilobite leg arises on the actual basal 

 segment of the limb, and the question has been raised as to whether 

 it is a true exopodite or an epipodite. If it is an exopodite homologous 

 with that of living crustaceans, then it throws the trilobite definitely 

 into the class Crustacea. If, on the other hand, it is an epipodite, then 

 it makes the trilobite ancestral to all the Arthropoda so far as the 

 structure of its legs is concerned. 



PART II. THE OSSICLES AND MUSCLES OF THE STOMACH 



Although it was not at first intended to do more than list the muscles 

 of the appendages, the structure of the stomach appeared to be so 

 mteresting that I have prepared a second part to my paper including 

 the muscles of the stomach and listing the ossicles on which they find 

 their attachment. The literature on the stomach muscles is even less 

 extensive than is that on the appendage muscles, and I find that some 

 of the muscles of the pyloric region of the stomach of decapod crus- 

 taceans have not been figured or described. 



It is logical to include the stomach muscles in the same paper with 

 the muscles of the appendages that originate on the body wall, for 

 developmental studies of invertebrates have demonstrated that the 

 gastric mill is merely an invaginated part of the body wall, so that 

 the muscles pertaining to it are as truly " skeletal " as are those of 

 the appendages. 



The word " stomach " is, as a matter of fact, a misnomer. The en- 

 larged part of the alimentary canal immediately following the esopha- 

 gus, although popularly referred to as a stomach, is a part of the 

 stomodaeum of arthropods and performs the same function as does 

 the gizzard in birds — ^that is, to pulverize the fragments of food and 

 render them small enough to be acted upon effectively by the true 

 digestive juices, which are secreted in the pylorus, a relatively small 

 section of the alimentary canal which follows the stomodaeum. 



But it is convenient to speak of the whole structure from the mouth 

 to the beginning of the intestine as the " stomach." As this has been 

 done in most of the preceding discussions by former authors, the term 

 has been used in the present discussion in the same broad sense. 



