774 



ZOOLOGY 



SECT. 



ciors 



, 



Tetrabranchiata there are, as already stated, in accordance with 

 the double number of gills, four auricles instead of two, and 

 branchial hearts are absent. 



Nervous system and Sense-organs. The ganglia of the 

 central nervous system are in all closely aggregated together round 

 the oesophagus, as already stated to be the case in Sepia ; and the 

 general disposition is the same as that described. In Octopus the 

 ganglia are much less sharply marked off. In Nautilus, as already 

 mentioned, there is less concentration, and distinct ganglia are not 

 recognisable. All the Dibranchiata possess highly developed eyes 

 similar to those of Sepia, elevated on stalks in certain cases ; but 

 in Nautilus the eyes are of a much simpler character, each con- 

 sisting of a sac opening on the exterior by a small rounded aperture, 

 lined internally by a two-layered retina similar to that of Sepia, 

 but without lens, vitreous humour, or cornea. In the embryo 

 of the Dibranchiata, the eye passes through a stage in which it is in 

 the condition of an open cup similar to the adult eye of Nautilus. 

 Osphradia are present, as already mentioned, only in the Tetra- 

 branchiata ; but in both the Dibranchiata and the Tetrabranchiata 



certain sensory processes or de- 

 pressions conjectured to possess 

 an olfactory function are de- 

 veloped on the head. Statoojsfs 

 are universally present. 



All the Dibranchiata have two 

 ^^ kidneys or renal sacs similar 



i^R p OS f in character to those of Sepia, and 

 communicating with one another ; 

 in Octopus they are completely 

 united. In the Tetrabranchiata 

 there are four kidneys, each open- 

 ing on the exterior. 



The sexes are distinct in all 

 the Cephalopoda, and in addition 

 FIG. TOG. segmenting ovum of Loiigo. to the hectocotylised arm there 



rler ' atterWatas6 - ) are .frequently other external 

 differences between male and female. In all the Dibranchiata 

 the arrangement of the gonads and gonoducts is, as regards general 

 features, similar to what we find in Sepia. In Octopus, however, 

 there are two oviducts instead of one. and in one other member 

 of the Octopoda (Eledone mosclmta) the same holds good of the 

 spermiducts. 



Development. The development of the Dibranchiata alone is 

 known. The eggs are very large, containing a relatively large 

 amount of food-yolk. They are usually laid in masses or strings 

 embedded in a soft, gelatinous, or a tougher, more leathery 

 substance, usually attached to some foreign body ; in some cases 



