250 



LOWER INVERTEBRATES. 



which brings the vent in another position will be discussed further on. The three 

 divisions of the digestive tract, stomodeum, mesenterou and proctodeum are well devel- 

 oped, the middle region being characterized by a very large liver. 

 Salivary glands are frequently present, emptying into the stomo- 

 deum, and the same region frequently bears a lingual ribbon, 

 armed with teeth, for the comminution of food. This organ is 

 employed to characterize the Cephalophora, one of the two great 

 divisions of the MoUusca, and will be described when treating of 

 that group. 



The ncn'vous system typically consists of two ganglia above 

 the oesophagus (cerebral) ; two at its sides (pleural) ; and two 

 beneath (pedal). These are connected by a ring of nervous 

 tissue. From each of the pedal ganglia arises a nerve cord which 

 ti-averses the length of the foot (the pedal nerve) while from the 

 pleural ganglia two similar cords arise, which also pass back- 

 ward, but at a higher level (the pleural commissures). These 

 terminate in a ganglion on either side, known indifferently as 

 the visceral or parieto-splanchnic ganglion. These two visceral 

 iiuiglia are connected with each other by a cord known as the 

 visceral loop, in the middle of which is the abdominal ganglion. 



- Diagram of ner- 



Fio. 235 

 vous .inatomy of inoUusc; 

 a, abilominal ganglion; 

 c, cerebral ganglion; 

 e, eye; 17, giU; /, pleu- 



pedai^fangiion ; "" ' OT- From the cerebral ganglia nerves go to the eyes, and primitively 

 fi'visoerai'gaiigi'ion.'^"^'' to the auditory organ. An additional commissure on either side 

 connects the cerebral with the pedal ganglion. 

 The heart, which is situated dorsally, consists of a ventricle and one or two auri- 

 cles. It is always arterial, receiving the blood from the respiratory organs and 

 forcing it to all parts of the body. The circulation is not completely closed, the 

 blood for a portion of its course flowing through channels without proper walls. 



Though the whole surface of the body has respiratory functions, special organs for 

 the aeration of the blood exist in the shape 

 of gills, or, less frequently, so-called lungs. 

 The gills are ciliated outgrowths from the 

 body, usually placed in the cavity of the 

 mantle between that envelope and the foot 

 or body wall. Each gill may be reduced 

 to a type called by Lankester a ctenidium. This, as its name indicates, is like a comb, 

 the back of the comb being the rhachis or stalk, while the gill lamelte correspond to 

 the teeth of the comb. In the rhachis are two canals, one carrying the blood to the 

 gill plates, there to be brought in contact with the water, the other returning it to the 

 heart. From this type most of the forms of gills can be derived. 



All of the gills of Mollusca are not homologous, a fact first pointed out by Spen- 

 gel. This anatomist has shown that in the true or typical gills are normally paired 

 organs, one or more being found on either side of the body. These true gills receive 

 their nerves from the visceral loop of the nervous system, and he has also pointed out 

 that at the base of each gill is a sense organ, the purpose of Avhich is to test by smell 

 the quality of the water supplied to each gill. This olfactory organ is also innervated 

 from the same part of the nervous sj'stem as are the true gills. Other respiratory 

 organs exist in some forms, but we have a sure test of their homology in their rela- 

 tions to the nervous system and to the organs of smell. 



Fig. 256. — Gill o£ Sei>ia. 



