ARTHROPODA 349 



31. The Heteropoda are pelagic Prosobranchia with foot divided into 

 fin and tail, shell rudimentary or absent. 



32. The Pidmonata are in some respects (orthoneurous and her- 

 maphroditic) Opisthobranch-like; in other respects as in position of 

 heart, development of shell and mantle like the Prosobranchs; the 

 mantle cavity connects with a lung. 



33. The CEPHALOPODA have no true foot; but its homologues are to be 

 found in the siphon and in the tentacles, usually provided with suckers, 

 on the head ; they have an unpaired mantle and mantle cavity and a single 

 shell or none. 



34. The mantle cavity contains one or two pairs of ctenidia. Water 

 is forced from the mantle cavity through the siphon. 



35. The number of auricles and nephridia corresponds with the number 

 of ctenidia; besides the systemic heart there are one or two pairs of branch- 

 ial hearts, elsewhere unknown in molluscs. 



36. The sexes are separate. 



37. The ink sac is peculiar to Cephalopoda. 



38. The eye is (usually) highly developed (with retina, chorioid, 

 iris, cornea, vitreous body, and lens), as is the nervous system, which has, 

 in addition to the usual centres, optic, sympathetic, and stellate ganglia. 



39. The eggs have a discoidal segmentation. 



40. The Cephalopoda are divided into Tetrabranchia and Dibranchia. 



41. The Tetrabranchia (extinct save for Nautilus) have four gills, 

 a chambered shell, primitive eyes, and finger-like cephalic lobes in place 

 of tentacles. 



42. The Dibranchia have two gills, eight or ten tentacles with suckers, 

 and the shell is reduced or absent. 



PHYLUM VII. ARTHROPODA. 



Under the term Arthropoda are included the crabs, spiders, insects, 

 and myriapods, which, together with the annelids, were united by Cuvirr 

 in his sub-kingdom Articulata. Annelids and arthropods agree in many 

 features. They are, as the term articulates implies, segmented animals, 

 and they differ from the vertebrates, which are also segmented, in the 

 extension of the segmentation, the ringing of the body, to the external 

 surface. The boundaries between the successive segments, which cannot 

 be recognized in the skin of the vertebrate, are marked in the articulates 

 by constrictions of the body wall, whence the old names evro/xa and 

 Insecta, applied to these forms. The articulates are further characterized 

 by a ladder-like nervous system (fig. 78), in which the brain is supple- 



