116 



MOLLUSCA 



termed, which is also the condition presented by the mouth- 

 bearing region in nearly all other Gastropoda. One of the 

 best examples of the introversible mouth-cylinder or pro- 

 boscis which can be found is that of the Common Whelk 

 and its immediate allies. In fig. 37 the proboscis is seen 

 in an everted state ; it is only so carried when feeding, 

 being withdrawn when the animal is at rest. Probably 

 its use is to enable the animal to introduce its rasping 

 and licking apparatus into very narrow apertures for the 

 purpose of feeding, e.g., into a small hole bored in the shell 

 of another Mollusc. 



The foot of the Azygobranchia, unlike the simple mus- 

 cular disc of the Isopleura and Zygobranchia, is very often 

 divided into lobes, a fore, middle, and hind lobe (pro-, 

 meso-, and meta-podium, see figs. 39 and 43). Very usually, 

 but not universally, the meta-podium carries an operculum. 

 The division of the foot into lobes is a simple case of that 

 much greater elaboration or breaking up into processes and 

 regions which it undergoes in the class Cephalopoda. Even 

 among some Gastropoda (viz., the Opisthobranchia), we 

 find the lobation of the foot still further carried out by 

 the development of lateral lobes, the epipodia, whilst there 

 are many Azygobranchia, on the other hand, in which the 

 foot has a simple oblong form without any trace of lobes. 



The development of the Azygobranchia from the egg has 

 been followed in several examples, e.g., Paludina, Purpura, 

 Nassa, Vermetus, Neritina. As in other Molluscan groups, 

 we find a wide variation in the early process of the forma- 

 tion of the first embryonic cells, and their arrangement as 

 a Diblastula dependent on the greater or less amount of 

 food-yelk which is present in the egg-cell when it com- 

 mences its embryonic changes. In fig. 7, the early stages 

 of Paludina vivipara are represented. There is but 

 very little food-material in the egg of this Azygobranch, 

 and consequently the Diblastula forms by invagination ; 

 the blastopore or orifice of invagination coincides with the 

 anus, and never closes entirely. A well-marked Trocho- 

 sphere is formed by the development of an equatorial 

 ciliated band; and subsequently, by the disproportionate 

 growth of the lower hemisphere, the Trochosphere becomes 

 a Veliger. The primitive shell-sac or shell-gland is well 

 marked at this stage, and the pharynx is seen as a new 

 ingrowth (the stomodaeum), about to fuse with and open 

 into the primitively invaginated arch-enteron (fig. 7, F). 



In other Azygobranchs (and such variations are repre- 

 sentative for all Mollusca, and not characteristic only of 

 Azygobranchia), we find that there is a very unequal 

 division of the egg-cell at the commencement of embryonic 

 development, as in Nassa (fig. 5). Consequently there is 

 strictly speaking no invagination (emboly), but an over- 

 growth (epiboly) of the smaller cells to enclose the larger. 

 The general features of this process and of the relation of 

 the blastopore to mouth and anus have been explained 

 above in treating of the development of Mollusca generally. 

 In such cases the blastopore may entirely close, and both 

 mouth and anus develop as new ingrowths (stomodaeum 

 and proctodseum), whilst, according to the observations of 

 Bobretzky, the closed blastopore may coincide in position 

 with the mouth in some instances (Nassa, &c.), instead of 

 with the anus. But in these epibolic forms, just as in the 

 embolic Paludina, the embryo proceeds to develop its cili- 

 ated band and shell-gland, passing through the earlier con- 

 dition of a Trochosphere to that of the Veliger. In the 

 veliger stage many Azygobranchia (Purpura, Nassa, &c.) 

 exhibit, in the dorsal region behind the head, a contractile 

 area of the body-wall. This acts as a larval heart, but 

 ceases to pulsate after a time. Similar rhythmically con- 

 tractile areas are found on the foot of the embryo Pulmo- 

 nate Limax and on the yelk-sac (distended foot-surface) 

 of the Cephalopod Loligo (see fig. 72**). 



The history of the shell in the development of Azygo- 

 branchia (and other Gastropods) is important. Just as 

 the primitive shell-sac aborts and gives place to a cap-like 

 or boat-like shell, so in some cases (Marsenia, Krohn) has 

 this first shell been observed to be shed, and a second shell 

 of different shape is formed beneath it. 



A detailed treatment of what is known of the histo- 

 genesis in relation to the cell-layers in these Mollusca would 

 take us far beyond the limits of this article, which aims at 

 exposing only the well-ascertained characteristic features 

 of the Mollusca and the various subordinate groups. There 

 is still a great deficiency in our knowledge of the develop- 

 ment of the Gastropoda, as indeed of all classes of animals. 

 The development of the gill (ctenidium) as well as of the 

 renal organ, and details as to the process of torsion of the 

 visceral hump, are still quite insufficiently known. 



One further feature of the development of the Azygobran- 

 chia deserves special mention. Many Gastropoda deposit 

 their eggs, after fertilization, enclosed in capsules ; others, as 

 Paludina, are viviparous ; others, again, as the Zygobranchia, 

 agree with the Lamellibranch Conchifera (the Bivalves) in 

 having simple exits for the ova without glandular walls, 

 and therefore discharge their eggs unenclosed in capsules 

 freely into the sea- water ; such unencapsuled eggs are 

 merely enclosed each in its own delicate chorion. When 

 egg-capsules are formed they are often of large size, have 

 tough walls, and in each capsule are several eggs floating 

 in a viscid fluid. In some cases all the eggs in a capsule 

 develop ; in other cases one egg only in a capsule (Neri- 

 tina), or a small proportion (Purpura, Buccinum), advance 

 in development ; the rest are arrested either after the first 

 process of cell-division (cleavage) or before that process. 

 The arrested embryos or eggs are then swallowed and 

 digested by those in the same capsule which have advanced 

 in development. The details of this history require renewed 

 study, our present knowledge of it being derived from the 

 works of Koren and Danielssen, Carpenter and Claparede. 

 In any case it is clearly the same process in essence as that 

 of the formation of a vitellogenous gland from part of the 

 primitive ovary, or of the feeding of an ovarian egg by 

 the absorption of neighbouring potential eggs ; but here 

 the period at which the sacrifice of one egg to another 

 takes place is somewhat late. What it is that determines 

 the arrest of some eggs and the progressive development 

 of others in the same capsule is at present unknown. 



Section b (of the Azygobranchia). NA TANTIA. 



Characters. Azygobranchiate Streptoneura which have the 

 form anil texture of the body adapted to a free-swimming pelagic 

 habit. They appear to be derived from holochlamydic forms of 

 Reptant Azygobranchia. The foot takes the form of a swimming 

 organ. The nervous system and sense-organs (eyes, otocysts, and 

 osphradium) are highly developed. The odontophore also is re- 

 markably developed, its admediau teeth being mobile, and it serves 

 as an efficient organ for attacking other pelagic forms upon which 

 the Natantia prey. The sexes are distinct as in all Streptoneura ; 

 and genital ducts and accessory glands and pouches are present as 

 in all Azygobranchia. The Natantia exhibit a series of modifica- 

 tions of the form and proportions of the visceral mass and foot, 

 leading from a condition readily comparable with that of a typical 

 Azygobranch such as Rostellaria, with the three regions of the foot 

 (pro-, meso-, and meta-podium) strongly marked, and a coiled 

 visceral hump of the usual proportions, up to a condition in which 

 the whole body is of a tapering cylindrical shape, the foot a plate- 

 like vertical fin, and the visceral hump almost completely atrophied. 

 Three steps of this modification may be distinguished as three sub- 

 orders, the Atlantacea, the Carinariacca, and the Ptcrotracheacea. 



Sub-order 1. Atlantacea. 



Characters. Natantia with a large spirally- wound visceral hump, 

 covered by a hyaline spiral shell ; mantle-skirt large, overhanging 

 a well-developed sub-pallial branchial chamber as in Azygobranchia, 

 to the wall of which is attached the branchial ctenidium ; foot 

 well developed, divisible into a mobile propodium, a mesopodium 

 on which is formed a sucker, and a metapodium which, when the 

 animal is expanded, extends backwards beyond the shell and visceral 



