NATURE OF THE INVERTEBRATE BRAIN. 29 



though ganglia exist around the oesophagus which must receive 

 afferent impressions of some kind, and from which branches proceed 

 to the various muscles and viscera of the body. 



Such low sensory endowments as are presented by the Brachiopoda 

 would be wholly incompatible with that degree of visceral complexity 

 of organization which they possess, had it not been for the fact that 

 they lead such a passive existence 'in respect to quest of food. They 

 do not go in search of it at all they remain securely anchored while 

 food is brought to the entrance of their alimentary canal by means 

 of cilia. The absence of sense-organs and of a brain is, indeed, only 

 compatible with a ^wtm-vegetative existence such as this. 



The lamellibranchs, or ordinary headless bivalve 3Iollusca, also 

 include some representatives such as the oyster and its allies which 

 lead a sedentary life after the fashion of the Mollusca already men- 

 tioned. The valves of the shell in these lamellibranchs are lateral, 

 instead of being dorsal and ventral as among the branchiopods. The 

 shell is, however, closed by a single adductor muscle, and it is opened, 

 when this relaxes, by means of an elastic hinge. 



The mouth of the oyster is surrounded by four labial appendages, 

 whose functions are not very definitely known. It presents no other 

 appendages of any kind in the neighborhood of the mouth, and, as in 

 the two types of Mollusca already described, the food which it swal- 

 lows is brought to the entrance of its oesophagus by means of ciliary 

 currents. This well-known animal has a large and important nervous 

 ganglion (Fig. 8, b) situated posteriorly, and close to the great ad 

 ductor muscle. It gives off' branches to this muscle, to each half of 

 the mantle, to the gills (c, c), and it sends forward two long parallel 

 branches (d, c7), which serve to connect it with a much smaller an- 

 terior ganglion (a, a) situated on each side of the mouth. These an- 

 terior or labial ganglia are joined by a commissure arching over the 

 mouth, and also by a more slender thread beneath the mouth, from 

 which filaments (e) are given off to the stomach. These latter filaments 

 may be considered to have a function similar to that of the stomato- 

 gastric nerves in insects. The anterior ganglia receive nerves (/) from 

 the labial processes, probably for the most part afferent in function. 

 At all events, these processes have no distinct muscular structure. 



Other lamellibranchs possess a remarkable muscular appendage 

 known as the foot, w*hich is in relation with an additional single or 

 double nervous ganglion, and is used in various ways as an organ of 

 locomotion. The animals possessing this organ are also provided 

 with a second adductor muscle for closing their shells. Speaking of 

 the various uses of the foot among bivalves, Prof. Owen says : " To 

 some which rise to the surface of the water it acts, by its expansion, 

 as a float ; to others it serves by its bent form as an instrument to 

 drag them along the sands ; to a third family it is a burrowing organ ; 

 to many it aids in the execution of short leaps." 



