PHYLUM MOLLUSCA 



789 



glands are present, but are mainly situated, as already pointed 

 out, on the posterior instead of the anterior wall of the mantle- 

 cavity. Each egg becomes enclosed 

 in an elaborate capsule (Fig. 686), 

 probably moulded by the agency of 

 the organ of Owen, on the inner ^^^'^iIn 



posterior lobe of the foot of the female ,^ftc\ 



(Fig. 677). The development is not 

 known. 4^ l\ ' / 1;! 



Fio. 680.— Egg of Nautilus 

 macromplialus, enclosed 

 in its c.ipsule. (Aiter Willey.) 



2 Distinctive Charactees and 

 Classification. 



The Cephalopoda are bilaterally 

 symmetrical Mollusca, which have the 

 main part of the foot displaced for- 

 wards to the neighbourhood of the 

 mouth and divided into a series of 

 arms bearing suckers, or of lobes 

 bearing tentacles, while the remainder 

 of the foot forms a funnel for the 

 egress of water from the mantle-cavity. 



The visceral mass is symmetrical and not coiled. The mantle 

 encloses posteriorly and ventrally a large mantle-cavity, in which 

 are situated the ctenidia and the nephridial, reproductive, and 

 anal apertures. The shell may be absent or rudimentary ; when 

 present and well developed, it may be internal or external, undivided, 

 or divided internally by septa into a series of chambers. There is 

 an internal cartilaginous skeleton, supporting and protecting the 

 nerve-centres and giving attachment to muscles. The mouth is 

 provided with a pair of horny jaws, and an odontophore is present. 

 In the majority there is an ink-gland with a duct opening into 

 the rectum. The ctenidia and nephridia are either two or four in 

 number. The nervous system is highly developed ; and the 

 principal nerve-ganglia are aggregated together around the 

 oesophagus. The sexes are separate ; the segmentation of the 

 ovum is meroblastic, and there is no metamorphosis. 



Sub-Class I.— Dibranchiata. 



Cephalopoda in which the main part of the foot assumes the 

 character of a circlet of either eight or ten arms, bearing suckers, 

 and surrounding the mouth. The funnel forms a complete tube. The 

 shell is usually internal ; when external its cavity is not divided by 

 septa. There are two ctenidia, two nephridia, and two branchio- 

 cardiac vessels or auricles. An ink-gland and duct are present. 



