MOLLUSCA. 257 



species have been described. Limax (Fig. 120) is a pulmonate form in 

 which the shell is practically wanting. It is especially destructive to cer- 

 tain types of plants as it is a voracious vegetable feeder. The aquatic 

 pulmonates are represented by the "pond-snail" (Limnaa), and by 

 Planorbis, a snail whose coils are in one plane, presenting a helix rather 

 than a spiral. 



Class III. Cephalopoda (Squid, Devil-fish). The cephalopods are 

 bilaterally symmetrical mollusks with a well-developed head in which 



FIG. 121. 



e' 



FIG. 121. Pearly Nautilus. From Nicholson, e, eye; h, hood, a muscular portion 

 of the foot which protects the softer parts; s, siphon; se, septa, separating the succes- 

 sive chambers of the shell; sp, siphuncle; t, tentacles. 



Questions on the figure. How does this shell compare with those of 

 the Gasteropods? What is considered to be the homology of the tentacles 

 or arms in Cephalopods? What is the siphuncle? What is the character 

 of the eye in Nautilus? 



the front part of the foot surrounds the well-armed mouth as a series 

 of lobes or tentacles. The head protrudes permanently from the mantle 

 cavity, leaving the mantle surrounding the posterior part of the body. The 

 posterior lobe of the foot forms a siphon, communicating with the 

 mantle cavity. Into this cavity the nephridia, the anus, and the reproduc- 

 tive glands open, and in it the gills lie. The shell may be present and ex- 

 ternal (Nautilus}, internal and slightly developed (Squid), or wanting 

 (Octopus). An internal cartilaginous skeleton protects the brain. The 

 coelom is well developed. The ganglia of the nervous system are massed 

 in the head region. The sexes are separate and the development direct. 

 The Cephalopoda are to be looked upon as the most highly developed of 

 the Mollusca. They are little in evidence now, however, as compared with 

 earlier times. 



Subclass I. Tetrabranchiata. Cephalopoda in which the front segment 

 of the foot is divided into lobes bearing numerous tentacles, without 

 18 



