Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/712

G84 MOLLUSCA whilst in some Gastropods, according to Bobretzky, they originate, as here shown, for Loligo. The egg-coverings of the Dibranchiate are very complete. Argonauta and Octopus deposit each egg in a firm oval case, thin and transparent, which has a long stalk by which (in Octopus) the egg is fixed in company with two or three hundred others to some foreign object. Sepia encloses each egg in a thick envelope of many layers resembling india-rubber. Loligo encloses many rows of eggs in a copious tough jelly, and affixes a dozen or twenty such egg-strings to one spot. Sepia and Loligo desert their eggs when laid. The female Octopus most jealously B mtf- Fio. 123. Right and left sections through embryos of Loligo. A. Same stage as fig. 121 (4). B. Same stage as fig. 121 (8) ; only the left side of the sections is drawn, and the food-material which occupies the space internal to the membrane ym is omitted, al, rectum ; is, ink-sac ; ep, outer cell-layer ; mes, middle cell-layer ; ym, deep cell-layer of fusiform cells (yelk-membrane) ; ng, optic nerve-ganglion ; ot, otocyst ; wb, the "white body" of the adult ocular capsule forming as an imagination of the outer cell-layer ; mtf, mantle-skirt ; g, gill ; ps, pen-sac or shell-sac, now closed ; dg, dorsal groove ; poc, primitive optic vesicle, now closed (see fig. 119) ; 7, lens ; r, retina ; soc, second or anterior optic chamber still open ; if, iridean folds. C. The primitive invagination to form one of the otocysts, as seen in fig. 121 (5) and (6). (After Lankester.) guards them, building a nest of stones and incubating. Argonauta carries hers with her in a special brood-holding shell. The development of the Pteropoda, so far as is known, presents no points of contact with that of the Siphonopoda rather than with that of the Gastropoda, owing to the fact that in them the egg has not an excess of food-yelk. Con sequently, we find typical trochosphere and veliger larvae among the Thecosomata (fig. 8, C, and fig. 81), whilst the isolated observation of Gegenbaur has made known very remarkable larvae referable to the Gymnosomata, and with little doubt to Pneumodermon (fig. 84). The former set of larvae are sufficient to demolish once for all the view which has been entertained by some zoologists, viz., that the velar disc of the veliger larva is the same thing as the ptero- podial lobes of the mid-foot of Pteropoda. The latter larvae are of importance in showing that, as in embryo Siphonopods so in embryo Pteropods, the sucker-bearing lobes of the fore-foot are truly podial structures, and only embrace the head and surround the mouth as the result of late embryonic growth. BRANCH K. LIPOCEPHALA. Characters. Mollusca with the head region undeveloped. No cephalic eyes are present ; the buccal cavity is devoid of biting, rasping, or prehensile organs. The animal is sessile, or endowed with very feeble locomotive powers. The Lipocephala comprise but one class, the Lamelli- branchia, also known as Elatobranchia and Conchifera. Class LAMELLIBRANCHIA. Characters. Lipocephala in which the archaic BILA TERAL SYMMETRY of the Mollusca is usually fully retained, and raised to a dominant feature of the organization by the lateral compression of the body and the development of the shell as two bilaterally symmetrical plates or valves cover ing each one side of the animal. The FOOT is commonly a simple cylindrical or ploughshare-shaped organ, used for boring in sand and mud, and more rarely presents a crawl ing disc similar to that of Gastropoda ; in some forms it is aborted. The paired CTENIDIA are very greatly developed right and left of the elongated body, and form the most prominent organ of the group. Their function is chiefly not respiratory but nutritive, since it is by the currents produced by their ciliated surface that food -particles are brought to the feebly-developed mouth and buccal cavity. The Lamellibranchia present as a whole a somewhat uniform structure, so that, although they are very numerous, it is not possible to divide them into well-marked sub-classes or sections, and orders. The chief points in which they vary are (1) in the structure of the ctenidia or branchial plates ; (2) in the presence of one or of two chief muscles, the fibres of which run across the animal s body from one valve of the shell to the other (adductors) ; (3) in the greater or less elaboration of the posterior portion of the mantle- skirt so as to form a pair of tubes, by one of which water is introduced into the sub-pallial chamber, whilst by the other it is expelled ; (4) in the perfect or deficient symmetry of the two valves of the shell and the connected soft parts, as compared with one another ; (5) in the development of the foot as a disc-like crawling organ (Area, Nucula, Pectun- culus, Trigonia, Lepton, Galeomma), as a simple plough- like or tongue-shaped organ (Unionacea, &c.), as a re-curved saltatory organ (Cardium, &c.), as a long burrowing cylin der (Solenacea, &c.), or its partial (Mytilacea) or even com plete abortion (Ostracea). The essential Molluscan organs are, with these excep tions, uniformly well developed. The MANTLE-SKIRT is always long, and hides the rest of the animal from view, its dependent margins meeting in the middle line below the ventral surface when the animal is retracted ; it is, as it were, slit in the median line before and behind so as to form two flaps, a right and a left ; on these the right and the left calcareous valves of the shell are borne respectively, connected by an uncalcified part of the shell called the ligament. In many embryo Lamellibranchs a centro-dorsal PRIMITIVE SHELL-GLAND or follicle has been detected (figs. 8 and 151). The MOUTH lies in the median line anteriorly, the ANUS in the median line posteriorly. Both CTENIDIA right and left are invariably present, the axis of each taking origin from the side of the body as in the schematic archi-Mollusc (see fig. 1 and fig. 131). A pair of NEPHRIDIA opening right and left, rather far forward on the sides of the body, are always present. Each opens by its internal extremity into the pericardium. A pair of GENITAL APERTURES, connected by genital ducts with the paired gonads, are found right and left near the nephridial pores, except in a few cases where the genital duct joins that of the nephridium (Spondylus). The sexes are often, but not always, distinct. No accessory glands or copulatory organs are ever present in Lamellibranchs. The ctenidia often act as brood-pouches. A dorsal contractile HEART, with symmetrical right and left auricles (fig. 143, A) receiving aerated blood from the

ctenidia and mantle-skirt, is present, being unequally de-