673 



MOLLUSC A. 



MOLOTHRU3. 



874 



become the lateral alse of the adult form, but are a production of the 

 anterior part of the epipodium, which usually disappears in the adult. 



Cephalopoda. lu this group the embryo attains a much higher develop- 

 ment before leaving the egg, and the modifications which its primary 

 form undergoes are extremely instructive. The first organs of the 

 Cephalopod which appear on the germ-disc are (fig. 12, iv. 1) the mantle, 

 which is simply a thickening in the middle of the hcemal surface with 

 somewhat raised edges ; around this is a surface representing the 

 mesosoma and foot, at one end of which is the mouth, and at the 

 other or anal extremity are placed two little processes, the rudiments 

 of the gills. Again, on each side of the mantle the mesosoma is pro- 

 duced into a longitudinal ridge occupying the precise position of the 

 epipodium. As development goes on, the haemal surface occupied by 

 the mantle grows out, and becomes a prominent sac, whose free edges 

 detaching themselves more and more for only a short distance ante- 

 riorly, but for almost the whole length of the sac posteriorly, give 

 rise to the mantle cavity (rv. 2). The intestine passing into the abdomen 

 thus formed becomes more and more bent upon itself, until at last it 

 makes a complete loop, open towards the neural side. With all this 

 the epipodium, remaining rudimentary in its anterior region, becomes a 

 free process on each side posteriorly (representing for a time the alae of 

 a Pteropod), but after a while these processes unite, and form a hollow 

 canal, the Funnel. The changes undergone by the margins of the 

 foot are not less remarkable ; they are produced from behind forwards 

 into four or five digitations on each side, the anterior pair of which 

 stretch in front of the mouth and unite over it; the digitations 

 elongate more and more, and the mouth is in consequence at last 

 placed in the centre of a sort of inverted cone, formed by the foot and 

 its prolongations the acetabuliferous arms (iv. 3). 



Such may be ta'ien as a very short abstract of Professor Kb'lliker's 

 most valuable ' Entwickelungs-Geschichte der Cephalopoden," and it is 

 needless to point out that it is our hypothetical process of modification 

 of the Archetype into the Cephalopod type, in other words. 



The Hcemal Mollusca. It is unnecessary to consider the develop- 

 ment of the separate families of these Molluscs, as the process, as far as 

 we know, is the same in all. We will take that of a Nudibranch 

 (Antiopa crutata) as a type, having recently had occasion to go over 

 it with especial reference to the points here under consideration. 



The end of the process of yelk-division (which, we may remark in 

 passing, results, not in the formation of ' nucleated cells," but simply 

 in that of smaller and smaller packets of yelk-granules) in this 

 Mollusc, is the formation of a blastodermic layer investing the remainder 

 of the yelk. The whole embryo next becomes more or less bell- 

 shaped, a sort of rim, with very long cilia, appearing at the broader end, 

 while a minute prominence is seen at the opposite extremity (in. 1). 

 A straight line drawn from this prominence to the centre of the surface, 

 surrounded by the rim, would have the body of the creature symme- 

 trically disposed around it. On the one surface is a deep pit, formed 

 by the edges of the blastodermic layer ; on the opposite a delicate trans- 

 parent cup, the rudiment of the future shell, and the indicator of the 

 position of the haemal surface and mantle, appears (in. 3). By degrees 

 the haemal surface becomes more and more prominent and the shell 

 larger. With this the prominence above referred to is thrust more 

 and more towards the right side, so that its position becomes quite 

 asymmetrical (it. 3, 5). At the same time the ciliated rim from being cir- 

 cular is produced laterally into a lobe on each side the ciliated lobes ; 

 the metapodium makes its appearance behind these as a small promi- 

 nence ; and a delicate operculum is formed upon the metapodium. The 

 aperture of the mouth may now be observed behind the ciliated lobes 

 and between them and the metapodium ; and the internal substance 

 of the germ is seen to present the outlines of an alimentary canal, 

 consisting of a rounded gastro-hepatie mass and a narrower intestine, 

 which turns abruptly forwards and upwards, to end on the right side 

 more or less hxmally in the before-mentioned prominence, whose 

 position has become thus extensively altered. The mantle cavity has 

 begun to appear as a sort of pushing-in of the integument around 

 the anal prominence. 



Two things are obvious in this series of developmental changes. In 

 the first place, the primary symmetrically of the embryo ; secondly, 

 the gradual asymmetry brought about by the development of that 

 portion of the body which bears the shell, and which is a portion of 

 the hicmal surface. 



Now this is perfectly in accordance with our hypothetical derivation 

 of the Haemal Molltaca from the Archetype, and the only point which 

 remains to be proved is, that this over-developed lucmal surface is to 

 be considered as a post-abdomen, that is, as a post-anal portion of the 

 h.otnal surface. 



This view has been taken in deriving these forms from the Archetype, 

 because it is much the more readily comprehensible, and has many 

 structural facts in its favour ; but we are by no means prepared to 

 assert that the post-anal position of the h;cmal outgrowth in the 

 Hiemal Molliuca may not be a secondary production, the result of a 

 gradual twisting to one side and backwards of a primarily pre-anal 

 outgrowth of the haemal surface. The facts just detailed with regard 

 to the development of Antiopa would favour this view ; but, on the 

 other hand, sufficient attention has not been paid to the process of 

 development of other Oatteropoda to decide whether it is in these 

 reapects identical with that of the Nudibranchs or not. The anatomy 



of adult Pectinibranchs and Pteropods would lead one to believe that 

 in these forms, at any rate, the hcemal flexure has been direct and 

 primary ; and it may be that a careful comparative study of develop- 

 ment of the Pectinibranchs and Nudibranchs will lead to the trans- 

 lation of the Nudibranchs to the Neural division, the final hiumal 

 flexure turning out to be a secondary modification. In the absence 

 of sufficiently conclusive studies of this kind, however, we prefer to be 

 guided by structural considerations, and thence to retain the Nudi- 

 branchs provisionally among the Molluscs with a haemal flexure. It 

 will probably be granted that the doctrine of a Common Plan among 

 the Mollusca, which has been advanced, will have its value as a guide 

 through the mazes of their varying organisation even although the 

 details of this first sketch should turn out to be even in many points 

 erroneous. 



MOLOCH. [DKACONINA.] 



MOLOSSUS. [CHEIROPTERA.] 



MO'LOTHRUS (Swaiuson), a genus of Birds placed in the sub- 

 family Icterince, under the family Sturnidce, with the following generic 

 character : Bill very short, thick, finch-like, conic, entire ; the culmen 

 not flattened, but slightly arched from the base, which ia rather ele- 

 vated. Wings lengthened, .pointed; the first quill longest. Tail 

 slightly rounded. Middle toe as long as the tarsus ; lateral toes of 

 equal length ; hind toe shorter than the tarsus. All the claws rather 

 small, and fully curved. 



M. Pecoria, Sw., the Cow-Pen-Bird, Cow-Blackbird, Cow-Troopial, 

 and Cow-Bunting (Icterus Pecoria, Temm. ; Emberiza Pecoria, Wils. ; 

 Frinyilla Pecoria, Gm.). 



Male : Head and neck brown, inclining to black; the rest of the 

 plumage shining black, glossy with violet reflections on the breast 

 and shot with greenish above ; irkles hazel ; legs and claws black. 



Cow-Bird (Molothrus Pecora). 

 a, mile ; b, fe:nile ; o, young. Wilsjn. 



