MOLLUSCA 



131 



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, 



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, ff.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 gUls 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 

 „ =„ T ^ , . ^i, ^ ,„,„.., nephridial apertures (fig. 101). The viscero- 



Fio. 89.— -Lateral view of the female Pearly Nautilus, contracted bv spirit and lyinff in its shell, • ti-lt. i./ -j ji. 



th& right half of which is cut away (from Gegenbaur, after Owenl. a, visceral hump; !!,por' pericardial chamber opens by tWO independent 



tionofthefreeedgeof the mantle-skirt reflected on to the shell, —the edge of the mantle-skirt anpr-tiirps to the extfirior and not into the 

 can be traced downwards and forwai-ds around the base of the lAid-foot o? siphon i;l,l, super- apertures TO tne exierior dnu uui mio me 



flcialoriginoftheretractormnscleof the mid-foot (siphon), more or less firmly attached to the nephridial sacS. ihere are tWO OVldUCtS 

 shcll,of which asmaU piece(s)is seen between the letters!,!; s(fartherbaok)pointsto the /_:„i,i „_j ipf4.\ in fVip fpmnlp anH two tsriprtn 

 Biphunoular pedicle, which is broken offshort and not continued, as in the perfect state, through (.ngnt and ielt; m tue lemaie ana two sperm- 



the whole length of the siphuncle of the shell, also marked s and s" ; points to the right eye ; ducts in the male, the left duct in both 



t is placed near the extremities of the contracted tentacles of the outer or annular lobe of the t . j • , 



fore-foot,— the Jointed tentacles are seen protruding a little from their long cylindrical sheaths ; v, sexes being rudimentary. 



the dorsal "hood" formed by an enlargement inthis region of the annular lobe of the fore- J^ large external shell either coiled or Straio'ht 



is present, and is not enclosed by reflexions of 

 mantle-skirt, except such narrow-mouthed shells as 



foot (m. in figs. 90, 91) ; V, a swelling of the mantle-skirt, indicating the position on its inner . 

 face of the nidamental gland (see flg. 101, g.n.). 



except in Nautilus, but other organs are present in the the 



Fio. 90.— Spirit specimen of female Pearly Nautilus, removed from its shell, 

 and seen from the antero-dorsal aspect (drawn from nature by A. G. 

 Bourne), m,, the dorsal "hood" formed by the enlargement of the outer or 

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

 cles (sr, g in flg. 88) ; n., 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 con-esponding 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 flg. 89, b ; the cup-like 

 fossa to which 6. and d. point in the present figure is occupied by the coil of 

 the shell ; sr.a. points to the lateral continuation of the nuchal plate 6. 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 



that of Gomphoceras, which were probably enclosed by the 



Fia. 91.— Lateral view of the same specimen as that drawn in flg. 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 ; t, 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 ; v, v, 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, olf). 

 Salivary glands are wanting. An ink-sac is not present. 

 Branchial hearts are not developed on the branchial adve- 

 hent vessels. 



