Pomona College, Claremont, California SI 



expanding. The hbcrs in this muscle, instead <>l running from the 

 origin to the insertion oi the muscle as a whole run dorsoventrally 

 from wall lo wall so that any stimulus acting on the nerve which 

 supplies this muscle and causing the fibers to contract would cause 

 the muscle to become rigid, oi less diameter and oi greater length. 

 Since the origin oi the muscle is in the back part oi the pharnyx and 

 since this organ is hound to the body wall by connective tissue and 

 muscles, the "expansion" ol the mandibular muscle must force the 

 mandible forward ami with it the forceps teeth ol the maxillae, which 

 are closely bound to it. I Ins action extrudes the teeth and a sec- 

 ondary reflex seems to be established that causes them to be snapped 

 together forcibly soon alter they are extruded. This reaction is 

 carried on with such vigor that I have known eunicids to bite them- 

 selves into two or three pieces while dying in fixatives. 



The advantage ol this extrusion system over the more primitive 

 proboscoideal eversion found in the forms without the denticular 

 pouch can be seen in the quickness ol the reaction, its superior vigor 

 ami the fact that the teeth are extruded first rather than as a final 

 consequence ol the comparatively slow eversion ol a soft fleshy 

 proboscis. 



SUMMARY 



1. The annelid, Aracoda semimaculata, is a highly specialized 

 and evolved member ol the lumbriconereidae, inhabiting the mussel 

 beds near I ,aguna. 



2. Reconstructions were made stereographically as described 

 in this paper. 



3. The brain is very complex and highly specialized sensorially. 

 It is divided into the sensor)', connective, nuchal and visceral sys- 

 tems, is symmetrica] and has a central tubal cavity running through 

 the lower part ol the main cross-connective portion ol the brain, 

 from front to back, parallel to a cavity which is partly filled with 

 muscles and glands, and runs between the visceral ganglion ami 

 the main brain ami forward between the dorsal ami ventral lobes ol 

 the lore brain. This central cavity, taken together with the very 

 complex and convoluted olifactory forebrain presents an appear- 

 ance which seems quite similar to that described by Patten in Limnlus 

 and other invertebrates, but which can probably be best explained 



