GASTROPODA ANISOPLEURA. 475 



pre-eminently muscular, and certain of its muscles the columellar muscles 

 connect the animal to its shell. The head, tentacles, buccal mass, and 

 even in some instances the nervous collar surrounding the latter, have 

 special retractor muscles, connected to or derived from the columellar 

 muscles. Calcareous spicules are found in the dorsal integument of Doris, 

 &c. among Non-Palliata. Thread cells (nematocysts) are found in sacs at 

 the apices of the cerata or dorsal processes, in some Non-Palliata, e. g. 

 Eolis, Tergipes. 



The cerebral ganglia are sometimes closely approximated, sometimes 

 far apart. The pleural and pedal ganglia usually lie below and at the sides 

 of the buccal mass, but are fused together surrounding the aorta cephalica 

 in stylommatophorous Pulmonata : and in the Non-Palliata the right and 

 left ganglia appear to be fused with the corresponding cerebral ganglia, but 

 remain connected by a slender loop beneath the oesophagus. In Fissurella 

 and Haliotis (Zygobranchia), in Turbo (Trochidae) the pedal ganglia are 

 replaced by two long cords extending down the foot, sheathed with 

 ganglion cells and connected by transverse c6mmissures. The pleural 

 ganglia are only incompletely separated from these cords anteriorly. The 

 ganglia or the cords replacing them are often of a deep orange colour. The 

 cerebro-pedal and cerebro-pleural connectives are sometimes of great length, 

 and distinct from one another ; sometimes short, and contained within a 

 common sheath. The two main pedal nerves are connected by transverse 

 commissures in Paludina among Azygobranchia and some Pulmonata (p. 

 3 20). The two visceral nerves connected with the pleural ganglia are 

 always united posteriorly, but the length of the loop thus formed and the 

 distinctness of its two visceral and the single abdominal ganglia are 

 variable. In one section of the Anisopleura, the Streptoneura, the posterior 

 union of the visceral nerves, which are always long, is situated dorsally to 

 the intestine, and the loop is therefore twisted with the torsion of the 

 visceral dome, the right side of the loop passing above, the left below the 

 intestine. In the other section, the Enthyneura, the union is situated 

 ventrally to the intestine, and the nerves are consequently not twisted, and 

 in this case the loop may be very short, yet the ganglia remain distinct, 

 e. g. in Limnaeus among basommatophorous Pulmonata. A pair of buccal 

 ganglia is invariably present, connected with the cerebral ganglia : but in 

 Fissurella, Haliotis, and Turbo their connectives have been traced to the 

 pleuro-pedal centres. Ganglia may also be present on other nerves. 



Sensory cells with sense-hairs occur on the tentacles of the head, and 

 of other parts of the body, and the body itself. A number of these cells 

 sometimes form projecting ' gustatory ' papillae. In Fissurella and certain 

 species of Trochus lateral organs, composed of a central mass of sense cells 

 with surrounding supporting cells, are found ventral to the tentacles of the 

 mantle furrow. Special ganglia are situate at the bases of these lateral 



