MOLLUSCA 



131 



Sense-organs .ire highly developed ; the eye exhibits a 

 very .special elaboration of structure in the Dlbranchiata, 



and a ivmurkable archaic form in the Nautilus. Otocysts 

 are present in all. The typical osphradiuni is not present, 



Fic SO.-Lateral view of the female Pearly Nautilus, contracted by spirit and lying in its shell, 

 the right half of which is cut away (from Gegenbaur, after Owen), o, visceral Simp; &,por- pericardial 



term liectocotylization is applied to this modification (see 

 tigs. ss, '.!.">, !ii>). Elaborate spennatophores 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, y.n,). 

 The egg of all Siphouopoda 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 



ne P hrid > al artures (fig. 101). The viscero- 

 Opens by two independent 



the male, the left duct in both 



fore-loot, tin- jointed tentacles are seen protruding a little from their long cylindrical sheaths' S6X6S being rudimentary. 



the dorsal "hood" formed by an enlargement in this region of the annular Inbe of the fore- A l arc rp p-rtprna] slip]] pitlipr rni'Wl nr strfli'crhr 

 ' A S 6 external Bn( el COUed Or Straight 



is present, and is not enclosed by reflexions of 



ne o e or 

 foot (m. in figs. 00, 01) ; I', a swelling of tin., mantle-skirt, indicating the position on its inn 



f the nidamental gland (see fig. ioi, j/.ii.). 



except in Nautilus, but other organs are present in the 



i. 



71. 



Fir.. 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). 7H., the dorsal "hood " formed by the enlargement of the outer i >r 

 annular lobe of the fore-foot, and corresponding to the sheaths of two tenta- 

 cles (ff, g in fig. 88) ; ., tentacular sheaths of lateral portion of the annular 

 lobe ; w., the left eye ; 6., the nuchal plate, continuous at its right and Irll, 

 posterior angles with the root of the mid-foot, and convspunilhig to tin- 

 nuchal cartilage of Sepia; c., visceral hump; it., 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, b ; the cup-like 

 fossa to which b. and d. point in the present figure is occupied by the coil of 

 the shell; /.. points to the lateral continuation of the nuchal plate '-. t<> 

 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 



the mantle-skirt, except such narrow-mouthed shells as 

 that of Gomphoceras, which were probably enclosed by the 



Flo. 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- pallia! chamber ; g, the mid-foot or 

 siphon; fc, the superficial origin of its retractor muscles closely applied to 

 tli.- shell and serving to hold the animal in its place ; 1, the siphuncular pedicle 

 of the visceral hump broken off short ; i', r, the superior and inferior ophthal- 

 mic tentacles. 



mantle as in the Dibranch Spirilla. 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 

 - HIM i-a), 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. 



