MOLLUSCA 



147 



whilst in some Gastropods, according to Bobretzky, they 

 originate, as here shown, for Loligo. 



The egg-coverings of the Dibranehiate 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 



Fro. 123.— Bight and left sections tluougli embryos of Loligo. A. Same stage 

 as fig. 121 (i). B. Same stage as fig. 121 (8) ; only the left side of the sections 

 is dl'awn, and the food-material which occupies the space internal to the 

 membrane ym is omitted, al, rectum ; is, ink-sae ; ep, outer cell-layer ; mes, 

 middle cell-layer ; ym, deep cell-layer of fusiform cells (yelli-membranej ; ng, 

 optic nerve-ganglion ; ot, otocyst ; w&, the "white body" of the adult ocular 

 capsule forming as an invagination of the outer cell-layer ; mtf, mantle-skirt ; 

 g, gill ; pSf 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 vidth 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 larvee 

 among the Thecosomata (fig. 8, C, and fig. 81), whilst the 

 isolated observation of Gegenbaur has made known very 

 remarkable larvse referable to the Gymnosomata, and with 

 little doubt to Pneumodermon (fig. 84). The former set of 

 larvse 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 

 larvse 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 B.—LIPOCEPHALA. 



Characters. — MoUusca 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 MoUusca 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 (XJnionacea, &c.), as a re-curved , 

 saltatory organ (Cardium, &c.), as a long burrowing cylin- 

 der (Solenacea, (fee), 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 motjth 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 sjrmmetrical right and 

 left auricles (fig. 143, A) receiving aerated blood from the 

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



