Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/696

Rh 668 MOLLUSCA [SIPHONOPODA, Sense-organs are highly developed ; the eye exhibits a very special elaboration of structure in the Dibranchiata, and a remarkable archaic form in the Nautilus. Otocysts are present in all The typical osphradium is not present, the mantle-skirt, indicating the position on its inner foot (TO. in figs. 90, 91) ; V, a swelling ^, ^^ ,... face of the uidamental gland (see fig. 101, &amp;lt;/..). except in Nautilus, but other organs are present in the Fro. 00. Spirit specimen of female Pearly Nautilus, removed from its shell, and seen from the antero-dorsal aspect (drawn from nature by A. G. Bourne), in., the dorsal &quot;hood&quot; formed by the enlargement of the outer or annular lobe of the fore-foot, and corresponding to the sheaths of two tenta cles (g, g in lig. 88) ; ?i., tentacular sheaths of lateral portion of the annular lobe ;. , the left eye ; b., the nuchal plate, continuous at its right and left posterior angles with the root of the mid-foot, and corresponding to the nuchal cartilage of Sepia ; c., visceral hump ; d., the free margin of the mantle-skirt, the middle letter d. points to that portion of the mantle-skirt which is reflected over a part of the shell as seen in fig. 89, 6 ; the cup-like fossa to which b. and d. point in the present figure is occupied by the coil of the shell ; g.a. points to the lateral continuation of the nuchal plate b. to join the root of the mid-foot or siphon. cephalic region, to which an olfactory function is ascribed both in Nautilus and in the other Siphonopoda. The gonads are always separated in male and female individuals. The genital aperture and duct is sometimes single, when it is the left ; sometimes the typical pair is developed right and left of the anus. The males of nearly all Siphonopoda have been shown to be characterized by a peculiar modification of the arm-like processes or lobes of the fore-foot, connected with the copulative function. The term hectocotylization is applied to this modification (see figs. 88, 95, 96). Elaborate spermatophores or sperm-ropes are formed by all Siphonopoda, and very usually the female possesses special capsule-forming and nidamental glands for providing envelopes to the eggs (fig. 101, g.n.}. The egg of all Siphonopoda is large, and the development is much modified by the presence of an excessive amount of food-material diffused in the protoplasm of the egg-cell. Trochosphere and veliger stages of development are conse quently not recognizable. The Siphonopoda are divisible into two orders, the names of which (due to Owen) de scribe the number of gill-plumes present ; but in fact there are several characters of as great importance as those derived from the gills by which the members of these two orders are separated from one another. Order 1. Tetrabranchiata ( = Schizosiphona, Tentaculifera). Characters. Siphonopodous Cephalopods in which the inrolled lateral margins of the mid-foot are not fused, but form a siphon by apposition (fig. 101). The circum-oral lobes of the fore-foot carry numerous sheathed ten tacles (not suckers) (fig. 88). There are two pairs of ctenidial gills (hence Tetrabranchiata), and two pairs of nephridia, consequently four nephridial apertures (fig. 101). The viscero- . pericardial chamber opens by two independent nephridial sacs. There are two oviducts (right and left) in the female and two sperm- ducts in the male, the left duct in both sexes being rudimentary. A large external shell either coiled or straight is present, and is not enclosed by reflexions of the mantle-skirt, except such narrow-mouthed shells as that of Gomphoceras, which were probably enclosed by the l FIG. 91. Lateral view of the same specimen as that drawn in fig. 90. Letters as in that figure with the following additions e points to the concave margin of the mantle-skirt leading into the sub-pallial chamber ; g, the mid-foot or siphon ; k, the superficial origin of its retractor muscles closely applied to the shell and serving to hold the animal in its place ; I, the siphuncular pedicle of the visceral hump broken off short ; r, r, the superior and inferior ophthal mic tentacles. mantle as in the Dibranch Spirula. The shell consists of a series of chambers, the last formed of which is occupied by the body of the animal, the hinder ones (successively deserted) containing gas (fig. 89). The pair of cephalic eyes are hollow chambers (fig. 118, A) opening to the exterior by minute orifices (pinhole camera), and devoid of refractive structures. A pair of osphradia are present at the base of the gills (fig. 101, off). Salivary glands are wanting. An ink-sac is not present. Branchial hearts are not developed on the branchial adve- hent vessels.
 * apertures to the exterior and not into the