of the Leech, we are warranted to bring in a verdict against it of conspiring the death of the animal that harboureth it, by sucking out its vital juices.

The preceding are external parasites: a more numerous host infest the viscera and intestines, but I intend to pass them over slightly, for they either do not influence the health and habits of the Mollusks they live and feed upon, or that influence has been unmarked.-Dujardin mentions two infusorial animalcules (Trichomonas limacis and Albertia vermiculus), whose habitat is the intestines of the slugs.* The lacustrine pulmonated Mollusca are more profusely verminous. More than one species of Distoma has been found in their viscera; and, to judge from analogy, we should suppose that the infected organs must suffer softening and disorganization. M. Bäer has also found a Filaria in the abdomen of Limneus stagnalis; and in many Mollusca of the same family he has met with a worm allied to the Naides living in the respiratory cavity, or hanging like little tufts of thread from the sides of the abdomen, whence he has named it Chotogaster. † Besides these a kind of animalcules named Cercariæ find an appropriate nidus for their evolutions in the body of the lacustrine snails; and the curious transmutations of form they undergo in the interior of the Mollusk, and in the circumfluent water, afford one of the most striking illustrations of Steenstrup's theory of alternating generations. ‡

The parasites of the fresh-water bivalves have furnished M. Bäer with subjects for a copious essay.§ One of them (Aspidogaster conchicola) lives in the pericardium of Unio pictorum, and of several Anodontæ; another worm was detected within the auricle of the heart swimming in the blood; and the species (Distoma duplicatum) to which this individual belonged swarmed populous in every part and viscus of the body of Anodonta ventricosa. Another parasite named Bucephalus polymorphus, from its likeness to an ox's head and its multiformity, breeds and multiplies in the liver and interior of these devoted mussels, which thus may

* Hist. Nat. des Infusoires, 300 and 654.

"La cavité respiratoire et le rein du Limneus stagnalis ont fourni un autre entozaire nouveau, rentrant, par son organisation, dans la classe des Annélides, et voisin des Naïdes. Les paquets de soies que ce ver porte par paires, sur les côtes de la surface abdominale, lui ont fait donner le nom de Chatogaster. Il s'est retrouvé dans le Planorbis corneus et dans beaucoup d'autres Mollusques d'eau douce. On les rencontre aussi à l'état libre dans les eaux habitées par ces Mollusques."

See Agassiz and Gould's Prin. of Zoology, i. 130. § Bull. des Sc. Nat. (Fev. 1829) xvi. 292–297.

be said to have something in common with royalty of the olden date,

The parasites of the marine bivalves, and of the marine Mollusca in general, have been scarcely indicated,* if we except an entozoon (Vertumnus tethydicola) that sucks to atrophy the beautiful Tethys, † and some that infest the Cephalopods. Delle Chiaie enumerates an Ascaris, a Filaria, a Scolex, a Cysticercus, a Monostoma, a Distoma, a Bothriocephalus, and a Dibothriorhyncus amongst the parasites of the latter class; and a peculiar species is devoted to the Argonaute.§ One of the most wonderful entozoa hitherto described is the leech-like worm, named by Cuvier Hecatostoma or Hecatacotyles, because of its having a hundred cups and upwards for attaching itself to its peculiar victim-the Octopus granulatus, one of the Cephalopods of the Mediterranean. This parasite "lives in the abdominal cavity, or even in the substance of the flesh of the polypus, the only animal which surpasses it in the number of cups with which it is furnished. M. Cuvier remarks how favourable this circumstance is to the metaphysicians who amuse themselves with composing the intestinal worms all of a piece with the elements furnished by the body of the animals which they inhabit. Here we have the body of a polypus, which has for its parasite a worm, so like the arm of a polypus, that the illusion cannot be greater. Of the two polypi which he produced before the Academy, there was one in which the Hexacotyles was attached to one of the arms, which it had even nearly destroyed, and which it seems in such a degree to replace, that at first sight it might be taken for the arm itself. 'Let it be judged,' said M. Cuvier, 'how many theories might be founded on such an extraordinary resemblance. Never has the imagination been exercised on so curious a subject.""|| I take my leave of it in the

* In his Entozoorum Synopsis, Berolini, 1819, Rudolphi does not mention a single species parasitical on these Mollusca.

"The characteristic Nudibrane of the Mediterranean, a giant among its tribe, Tethys leporina, was only met with once, swimming foot up on the surface of the sea in the Gulf of Smyrna in an exhausted state, its sides being invested by that extraordinary parasite the Vertumnus tethydicola." E. FORBES in Report Brit. Assoc. 1843, p. 133.

Anim. s. Vert. Nap. iv. 61: 200, 201.

Tricocephalus acetabularis. D. Chiaie, lib. cit. ii. 225.

Edinb. New Phil. Journ. Jan. 1830, p. 102. Edinb. Journ. of Science,

i. 219.

words of a favourite author: "Surely as Samson at his marriage propounded a riddle to his companions to try their wits thereon; so God offereth such enigmas in nature, partly that men may make use of their admiring as well as of their understanding; partly that philosophers may be taught their distance betwixt themselves, who are but the lovers, and God, who is the giver of wisdom."

*

* * "The sea is His, and He made it."-PSALMIST.

Verily, for mine own part, the more I looke into Nature's workes, the sooner am I induced to beleeve of her even those things that seem incredible."-PLINY.

66

Quapropter, quæso, ne nostra legentes, quoniam ex his spernunt multa, etiam relata fastidio damnent, quum in contemplatione Naturæ nihil possit videri supervacuum."-PLINIUS.

385

LETTER XXII.

ON THE STRUCTURE AND FORMATION OF SHELLS.

It

IN each of the great divisions of the animal kingdom recognised by zoologists, there is a slightly organised part which, because of its more solid structure, seems intended to protect the vital organs from injury, and to afford points of attachment to the muscles that move the body. In the vertebrated and annulose sub-kingdoms this part or skeleton consists of many pieces articulated by plain sutures or by moveable joints, and those destined to be the fulcra of the muscles of locomotion are placed in pairs on the sides; but while the skeleton of the vertebrates is interior and clothed with the muscles, that of the annulose animals is a mere hardening of the skin, and, in many instances, is shed and replaced by a new one, formed under the older, more than once during the insect's or worm's progress to maturity. The analogous part in the Mollusca resembles the skin also in being commonly exterior in its position. is either mucous or coriaceous in texture, or oftener solid and calcareous, consisting of one or two unjointed pieces; but, unlike the skin of the Annulosa, it is not deciduous nor divided into annular segments, and has no closer connexion with the soft parts underneath than what is given through one or two muscles that connect the body with it at one or two points only, yet in such a way that their union during life is indissoluble. The skeleton of the vertebrates is, in fact, an organized portion of the body, penetrated with bloodvessels and absorbents, and, like other organized parts, ever undergoing change; but the shell of the Molluscans, although+ the result of life and organic change, becomes extravascular, so that when once formed it is incapable of being altered by any power inherent in itself. There is this further difference between the skeleton and the shell:-the former is most important in its relations to the function of locomotion, the latter in its use as a covering and shield,* for you will

*M. Robineau-Desvoidy thinks the shell of the Mollusca the analogue of the vertebral apparatus in higher organisms, but his speculations are quite unintelligible to me. The reader will find them in the Edinb. Journ. Nat. and Geogr. Sc. ii. 222, &c.

C C

remember that the central foot, the principal organ of progression in the Mollusca, has either no connection with the shell or an indirect and remote one. Hence we find that when the shell is too small to contain and shelter the entire body it is placed over the heart and respiratory organs, to protect from hurt those, beyond question, most essential to the animal's existence.

Such a shell, however, is not the exclusive possession of the Mollusca. I do not mean to compare with it the testaceous covering of the acorn-shells or of the barnacles (Cirrhopoda), for the differences between them in their composition or combination of parts are so great as to render their distinction easy; but there is a genus of crustaceous insects (Cypris) whose covering very exactly resembles the shell of a minute bivalved Mollusk ;* and without a knowledge of the inhabitants it must be difficult, if not impracticable, to distinguish the calcareous case of some marine worms from the tubular shells of certain Gasteropods. The sameness is so near that the best naturalists misarranged the latter until the animals of them were discovered; and I may just remind you that the microscopical chambered shells, long believed to be the productions of Cephalopods of commensurate diminutiveness, are now more justly considered to belong some of them to the Annelides, and others of them to much inferior organisms. To establish a line of demarcation between these productions and the shells of the Mollusca is, perhaps, important in a geological view, but I am not aware of any that can be in every case made apparent. The points in the interior of the shell on which the muscles of the Mollusca are inserted receive a mark or impression from them which is ever afterwards retained, and when such marks are observed, the conclusion of the shell being a Mollusk's is very certain, for no Annelide is fixed within its tube, but resides within at liberty to leave it on an emergency, and hence no constant marks on the parietes are ever made. But these marks, even when existing, may be too faint to be always perceived, or the smallness of the shell may present an insurmountable barrier to the search of the test; and in this dilemma I know of no other that you can unhesitatingly depend upon.

"The great affinity that the coverings, or shells, of some of this class of insects bear to the testacea tribe, has in all probability caused many to have been considered as small species of Mytilus, or the fry of larger; for many such Monoculi are capable of shutting their valves entirely, and inclosing every part of the animal; in which state they always are when dead, so that it is no easy matter to discriminate."- Montagu Test. Brit. 174. + Dujardin, Hist. Nat. des Infusories, p. 240-45.