452 THE ANIMAL KINGDOM. 



in the Medusa Charybdaea^. The otocysts nearly always lie close to the 

 pedal ganglia. They generally arise from the epiblast as pits which 

 close forming a vesicle, sometimes as solid ingrowths. They contain one 

 or more calcareous otoliths, and are lined by an epithelium which is 

 generally ciliated and occasionally provided with sense-hairs. 



The digestive tract is composed of a stomodaeum, a mesenteron, 

 and in some instances at least a proctodaeum. The stomodaeum in the 

 Glossophora is muscular, has appended salivary glands, and contains an 

 organ known as radula, composed of a chitinous membrane bearing 

 chitinoid teeth, developed within a sac (radular or odontophore sac), and 

 growing throughout life. It is borne in turn upon a subradular membrane 

 and cartilages, the latter always, the former sometimes, moved by muscles, 

 the whole constituting the odontophore. The shape and arrangement of 

 the teeth vary much; for development, &c., see p. 115. The mesenteron 

 is of some length. It is rarely straight, and its disposition is affected by 

 the mode of growth of the visceral dome. It makes a single simple bend 

 upon itself in Cephalopoda and Pteropoda, the concavity of the bend being 

 turned to the pedal ganglia. In other Mollusca it is more or less coiled. 

 A stomachal widening can generally be distinguished. A liver is always 

 present in the shape of either simple caeca appended to the stomach or 

 of a large gland, paired except in Cephalopoda. The cells of the liver 

 follicles are of three kinds granule cells, ferment and lime-producing 

 cells. The first-named are always present. Ferment-cells are said to 

 be absent in Pteropoda, whilst the last-named are absent in Lamelli- 



1 Patten has very recently published a most important paper on the eyes of Molluscs and Ar- 

 thropods (Mitth. Zool. Stat. Naples, vi. 1886). He points out on pp. 544-5 the following general 

 features. The Molluscan hypodermis, especially where exposed to light, has a cuticula divisible 

 into an outer structureless layer, the corneal cuticula, and an inner retinidial cuticula in which 

 ramify the terminations or retinidia of the hypodermis nerves. Every eye consists as a rule of a 

 number of eye-elements or ommatidia, which may and do occur isolated as well as aggregated. 

 Every ommatidium is composed of 2-4 central cells or retinophorae fused together, and inclosing an 

 axial nerve, and of one or more surrounding circles of pigmented cells or retinulae. In the more 

 primitive instances both retinophorae and retinulae terminate, each cell in its own cuticular rod, 

 with a retinidium or nervous rete ; in more specialised instances the retinulae do not possess rods. 

 A collection of ommatidia in which the rods of the retinophorae and retinulae, or of the former 

 alone, constitute a continuous layer, and the retinulae retain their pigment and position surrounding 

 the retinophorae, is to be termed a retineum. When the retinophorae alone retain their rods, and 

 each group of retinophoral cells is completely isolated, the resulting structure is an ommateum as in 

 the compound eyes of certain Lamellibranchiata and Arthropoda. If the retinulae belonging to 

 each ommatidium lose not only their rods but also their pigment, and are transformed into ganglion 

 cells, then the collection of eye-elements constitutes a retina as in Vertebrata, Pecten, Spondylus, 

 and Onchidium(l}. It may be added that the retinophorae terminate in a fibre continuous with 

 a nerve ; the retinulae in root-like fibres which enter the basement membrane of the hypodermis ; 

 that the former contain nuclei, one of which is large, the latter a nucleus which is non-nucleolate. 

 The nerve-fibres of the retinulae lie between the cells, as is the case in the hypodermis at large ; 

 whilst the retinophorae not only contain an axial nerve-fibre, but have others surrounding them 

 externally. The axial nerve-fibres form a rete in the rods. The greatest number of types of eye are 

 found in the Lamellibranch Area. 



