XII 



PHYLUM MOLLUSCA 



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 

 posterior lobe of the foot of the female 

 (Fig. 677). The development is not 

 known. 



Fio. GSG. Egg of Nautilus 

 macromphalus, enclosed 

 in its capsule. (After Willcy.) 



2 DISTINCTIVE CHARACTERS 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. 



