Ferment-cells are said to be absent in Pteropoda, whilst the last-named are absent in Lamellibranchiata and Pteropoda, and some Gastropoda (see pp. II 6-7). The anus opens into the sub-pallial space. Its typical position is posterior and always above the foot. In one division of Gastropoda, the Anisopleura, where torsion of the visceral dome has taken place, it is either on the right side or anterior. It has the same anterior position, either on the right or left side, in Pteropoda.
1 Patten has very recently published a most important paper on the eyes of Molluscs and Arthropods (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 (?). 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 Arca.
All Mollusca are furnished with a heart except the Scaphopoda. This heart consists of a thick-walled ventricle giving off an aorta single or double, and receiving blood from a single (most Gastropoda, Pteropoda) or double (i. e. right and left) auricle, in both cases obviously derived from a special dilatation and thickening of the branchial veins bringing back the blood from the ctenidia. But in Nautilus (Cephalopoda Tetrabranchiata), the only Molluscan with four gills, and therefore four efferent branchial veins, the four auricular dilatations are not well marked, and scarcely if at all thickened. The arterial system of vessels is very well developed, and capillaries are present in many Cephalopoda and some Pulmonate Gastropoda, but as a rule the blood-system is to a great extent lacunar. Sinuses or veins with defined walls usually convey the blood to the ctenidia. Amoeboid corpuscles are always present. Haemoglobin is very rarely found. Haemocyanin - a copper-containing pigment, blue when oxydised, colourless when deoxidised, and acting as an oxygen-carrier, has been detected in various Mollusca. The heart is lodged in a pericardium or secondary coelome.
It is apparently a closed off portion of the coelome which is not filled with blood and communicates with the exterior through the nephridia, in Nautilus alone by independent openings. The rest of the coelome has the form of irregular spaces filled with blood and lodging the viscera. The entrance of water into the blood-system, either through special pores, inter-epidermic channels, or through the nephridia and the pericardium, appears to be very questionable. A 'pericardial gland' which appears to have an excretory function is present in Cephalopoda, some Gastropoda and various Lamellibranchiata.
There are four nephridia or renal sacs in Nautilus, two in other Mollusca, with the exception of the Pteropoda and the majority of the Gastropoda Anisopleura, in which one sac is aborted. They communicate internally with the pericardium and open externally near the anus, except in Lamellibranchiata where the apertures are anterior. The nephridial sac is often differentiated into a non-secreting duct, and a glandular portion which is frequently dark-coloured from the presence of excretory products in the epithelium. It is ciliated internally except in Cephalopoda, and the ciliary currents appear to set outwards 1.
1 Cells of a whitish colour and containing uric acid are found in lines between the coils of the intestine in Cyclostoma elegans, forming the 'Concrement-driise;' Barfurth, Z. A. vii. 1884. Cells either pigmented or not pigmented, and containing uric acid are found scattered as a 'diffuse
The sexes are united in Pteropoda, euthyneurous Gastropoda Aniso-pleura, and a few Lamellibranchiata. Among the last-named different parts of the same sexual gland may be male or female; or, as in Ostrea edulis and the other groups referred to, there is an hermaphrodite gland in which sperm and ova develope near to one another, or side by side. The glands themselves are simple in structure. In Cephalopoda they are developed in the walls of the pericardium, as they appear to be in the Gastropodan Neomeniae and Chaetoderma. In these latter the genital products are set free through the nephridia, in Cephalopoda by special ducts. The glands in other Mollusca are continuous with their own ducts, but in some Lamellibranchiata the latter may open into a common sinus with the nephridial ducts, or as in Scaphopoda into the nephridium. There is no accessory genital apparatus of any kind in Lamellibranchiata and Scaphopoda. But other Mollusca possess at least an intromittent organ, in Cephalopoda a modified arm, in others a process of the body-walls, either remote from or connected to the genital aperture; a gland which secretes albumen for the ova; and in some cases organs such as a uterine dilatation, receptaculum seminis, vesicula seminalis, together with special glands.