. Cephalopoda. Cephalopoda. 9Jfx^ FIGURE 109. Relationship of the vena cava to the hind intestine in different Dibranchiata: b) typical condition, permanent in the Sepiolidae, otherwise found only on the embryo: - vena cava; - left arm; — right arm, receiving the right mesenteric vein (v. ms) from the depth and joined by the genital vein (); — abdominal vein; — pericardial gland, a) corresponding condition in the Oegopsida; cj in the Octopoda. Of particular importance for the characterization of the group are the luminous organs, a new development which plays a maj
. Cephalopoda. Cephalopoda. 9Jfx^ FIGURE 109. Relationship of the vena cava to the hind intestine in different Dibranchiata: b) typical condition, permanent in the Sepiolidae, otherwise found only on the embryo: - vena cava; - left arm; — right arm, receiving the right mesenteric vein (v. ms) from the depth and joined by the genital vein (); — abdominal vein; — pericardial gland, a) corresponding condition in the Oegopsida; cj in the Octopoda. Of particular importance for the characterization of the group are the luminous organs, a new development which plays a major part in its ecology and classification. The typical windows on the head of the Loliginidae () and Oegopsida () suggest that their common ancestral forms () already had luminous organs on the eyeballs. However, there is no proof of the primary existence of these organs, but we have to attribute them to the Oegopsida in particular: luminous organs are typical for the group, and not only at the point mentioned but widely distributed on the body. These organs are differentiated areas of skin where luminous secretions are produced and activated by auxiliary mechanisms. They are perhaps organs for the 231 culture of luminous bacteria as Pierantoni (1918) suggested for the Sepiolidae (q. v.)''' It must be assumed, at any rate, that the ancestral form of all Oegopsida had very simple luminous organs which were distributed more widely than is observed in any recent species. It is difficult to decide whether the luminous organs developed from the accessory nidamental glands (cf. p. 126 and the Sepiolidae). The isolated occurrences within the Oegopsida can be interpreted by differentiation and specialization. Together with Pfeffer (1912, pp. X and XI) we assume the following categories of luminous organs in the Oegopsida: 1. Organs of the surface, in form of small warts scattered on mantle, head and arms, especially dense on the ventral side (e. g, Enoploteuthidae)
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