2IO ZOOLOGY 



the shell, it terminates in a trifid lobe. Dorsal to it is 

 the oral cone or head, at the end of which the mouth opens, 

 surrounded by short pinnate tentacles (Fig. 123). A buccal 

 mass and a radula are present ; two liver lobes symmetrically 

 placed open into the stomach, from which the intestine passes 

 to open by the anus in the ventral middle line. 



A right and left nephridium are present, and open to the 

 exterior on either side of the anus. There is no heart, but 

 the coelom contains a colourless blood. At the base of the 

 oral cone a number of ctenidial filaments have their origin. 

 These are capable of very considerable extension. The nervous 

 system consists of a pair of cerebral ganglia, close to which lie 

 the pleural ganglia. Long commissures connect the cerebral 

 with the pedal ganglia. The visceral commissure is also long, 

 and bears the olfactory ganglia, situated in front of the anus, in 

 the same position as in Lamellibranchs. The generative gland is 

 alike in both sexes ; it is situated dorsally, and its products 

 make their exit through the right nephridium. 



Class Cephalopoda. 



Chaeacteristics. — BUatercdly symmetrical Glossaphora. The 

 visceral hu/nvp is elongated, not twisted ; the sub-pallial chamber 

 is chiefly developed posteriorly, and contains the gills, anus, 

 and excretory pores. The shell may he external or internal, 

 in a few cases it is absent. The foot has grown round the 

 head, and is hrohen up into the characteristic arms of the 

 Cephalopoda, provided with siickers. Part of the foot forms 

 a funnel-like siphon, which guides the water as it is expelled 

 from the pallial cavity. The vascular system is well 

 developed, in addition to the central heart consisting of a 

 ventricle and two auricles, an accessory branchial heart exists 

 at tJte base of each gill in all but Nautilus. Powerful beak- 

 like homy or calcareous jaws guard the mxmth, and the 

 radula is well developed. Chromatophores are present in the 

 integument. The Cephalopoda are dioecious. 

 The Cephalopoda are divided into two orders : (i.) The 



Tetrabeanchiata or the Tentaculifera and (ii.) the Di- 



BEANCHIATA or the ACETABULIFERA. 



