Part 12 (1/2)
A, Nauplius, just hatched; B, young fee of fe ; F, fully-developed feills of Cod, Haddock, and other common fish, we often find a red wor 73, F), which at first sight seems to bear no sort of resemblance to a Crustacean The soft body is curiously doubled up, and is attached to the host by a narrow neck; while dissection will reveal a s three branched outgrowths, which penetrate into the surrounding tissues and make the attachment of the parasite more secure Near the hinder end of the body are two coiled threads, which are the egg-es of the swi feet may be detected on and near the head, but apart from these it would be hard to find any characters to show that the animal is a Crustacean
The life-history of _Lernaea_ is very re 73, A), and after passing through soes they becoh, however, they choose a very different host fro 73, B) they attach theills of one of the Flat-fishes (Pleuronectidae), such as the Flounder, Plaice, etc, attachland siidae, already mentioned
The anies beco soain acquires the power of swi, and leaves its host Both sexes beco 73, C, D), and i further, but the females seek a second host, a fish of the fa on the gills, beco 73, E) into the adult forill cavities of the strange-looking fish known as the Angler or Fishi+ng-frog (_Lophius piscatorius_) there may often be found speciibbosus_ It has a soft, unseg, provided with nuular shape On the under-side, near the front, are forked lobes representing two pairs of the swi feet At the hinder end are usually attached a pair of long thread-like egg--masses are attached, close inspection of the under-side of the body will reveal a very ot-like object This is a male individual, which is attached, like a secondary parasite, to the body of the enorer female
[Illustration: FIG 74--STAGES IN THE LIFE-HISTORY OF _Haemocera danae_, ONE OF THE MONSTRILLIDae (Froy,” after Malaquin)
A, Free-swi into the body of the wores in the body of the host; F, free-swied, not to same scale) _a'_, Antennule; _br_, brain; _e_, nauplius eye; _f_, swis are carried; _m_, position of mouth; _md_, hooked s carried by female; _ovy_, ovary; _pr_, absorptive processes]
In all the cases mentioned, the animal is parasitic in the final state of its existence--at least in the female sex--but there are a few Copepoda which are free-swi and when adult, but parasitic in the inter the Copepoda taken by the tow-net in British seas, there may so 74, F), which are rees between the antennules and the first pair of swi feet
They have no trace of jaws, and only a ; while internally there is no food-canal, so that the ani nourish a reater part of their life is passed as internal parasites in certain bristle-footed wor 74, A) withoutthe third pair of appendages ( hooks, by means of which they fasten on to the hich is to serve as their host The nauplius bores through the skin of the wores in the process, andits way into one of the bloodvessels in the for 74, B), within which no organs except the degenerating nauplius eye can be detected It later beco finger-like processes grow out, which are believed to have the function of absorbing nourish 74, C, D) Within the cuticle the organs of the adult ani 74, E), and when fully forh the tissues of its host bythe pointed posterior end of the sac On reaching the surface the enclosing membrane bursts, and the adult animal is set free
Of all Crustacean parasites, however, perhaps the most remarkable in their structure and life-history are the Cirripedes of the order Rhizocephala It is not uncommon on the British coasts to find specimens of the common Shore Crab or other Crabs which carry under the abdomen an oval fleshy body This is the Rhizocephalan _Sacculina carcini_ (Plate XXIX), and it would hardly be possible to guess, from its appearance or structure, that it was a Cirripede or a Crustacean at all It is attached to the under-side of the Crab's abdomen by a short stalk, and in thewhich leads into a cavity separating the outer ”mantle” from the body of the animal
Very often this s enclosed in sausage-shaped packets At the point where the short stalk enters the abdoives off an ihout the body of the Crab, and even into its legs and other appendages By means of these roots the _Sacculina_ absorbs nourishment from the body-fluids of its host Like most Cirripedes, _Sacculina_ is hermaphrodite, and the body within the ans of the two sexes and a s the whole of the nervous system There is no es Another Rhizocephalan, _Peltogaster_, is not uncoh the nauplius larva of _Sacculina_ was described, and its reseo as 1836, by that acute observer, J Vaughan Thompson, it is only recently that the full life-history has been e and Mr Geoffrey S 75, A) resembles that of the normal Cirripedes, especially in the shape of the dorsal shi+eld, which is drawn out on either side in front into a pair of fronto-lateral horns It has, however, no mouth, and the food-canal is quite absent As in the nore (Fig 75, B), also mouthless, and it is in this form that the _Sacculina_ seeks the Crab on which it is to become parasitic It would be almost impossible for the _cypris_ larva to settle on that part of the Crab where the adult _Sacculina_ is afterwards to appear, since the Crab usually has its abdoainst the under-side of its thorax The larva therefore attaches itself on soing to a hair by h the cuticle at the base of the hair, and the contents of its body pass into the interior of the Crab as a littlecast off This mass of cells, which constitutes the embryo _Sacculina_, is carried about by the blood-currents of the Crab till it reaches the under-side of the intestine, where it beco 76), and as it grows the centralthe intestine of the Crab till it reaches the place where the adult parasite is to eans of the _Sacculina_ beco tissues between it and the external cuticle to degenerate, so that when the Crab h which the body of the parasite protrudes Owing, no doubt, to the drain on its systerow, and it does notas the parasite remains alive
[Illustration: FIG 75--FREE-SWIMMING STAGES OF _Sacculina carcini_
MUCH ENLARGED (After Delage)
A, Nauplius; B, cypris stage]
[Illustration: FIG 76--EARLY STAGE OF _Sacculina_ WITHIN THE BODY OF A CRAB (After G Smith)
_i_, Intestine of the Crab; _s_, body of the _Sacculina_, which afterwards ees on the under-surface of the Crab's abdomen; _r_, roots of the _Sacculina_]
In addition to this arrest of growth, _Sacculina_ produces in its hosts other changes, which affect chiefly the reproductive organs and the structures associated therewith Crabs of either sex infected with _Sacculina_ are incapable of breeding; the genital gland (ovary or testis) is found on dissection to be shrivelled up, and the external characters indicative of sex becoes have been most fully studied in the case of a kind of Spider Crab common at Naples--_Inachus mauritanicus_ In this species it is found that females infected with _Sacculina_ show no conspicuous external es, which in the norreatly reduced in size Infected ree the characters proper to the fee, except that the chelipeds re males Other specimens have, in addition, the abdomen much broader than in normal males, and sometimes as broad as in the females Finally, some males develop on the abdoes on the first and second soes on the next three somites, as in the females; these individuals are, in fact, so completely intermediate in character between the two sexes that it is only by dissection that it is possible to recognize them as modified enerate Rhizocephala have been derived froiven by a peculiar species of pedunculate Barnacle, _Anelasfish in the North Sea In _Anelasma_ the peduncle becomes deeply buried in the flesh of the Shark, and its surface is covered with short branching, root-like filaments As in the case of the Rhizocephala, these roots appear to absorb nutrih _Anelasma_ possesses a food-canal and mouth, the cirri are reduced in size and devoid of hairs, so that they cannot be used for obtaining food as in ordinary Barnacles
CHAPTER XI
CRUSTACEA IN RELATION TO MAN
The Crustacea come into relation with human life in the most obvious and direct way in the case of those species that are used for food The number of species so used in various parts of the world is very large, al that the species shall be sufficiently large and abundant to er Crustacea belong to the Decapoda, it is this order that supplies practically all the edible species, al a few Barnacles which are eaten in various parts of the world Thus the sessile Barnacle _Balanus psittacus_, found on the coasts of Chili, and growing to a length of 9 inches by 2 or 3 inches dia to statements quoted by Darwin, ”universally esteemed as a delicious article of food,” and the pedunculate _Pollicipes cornucopia_ is used for food on the coasts of Brittany and Spain
By far the most valuable of all the edible Crustacea are the European and Aammarus_ and _H americanus_) The former is found on the coasts of Europe fro mostly a short distance beloater mark wherever the botto, Lobsters are common on a sandy bottom, but as a rule they seeh hard bottoht in traps known as ”Lobster pots” or ”creels,” which vary in construction in different localities
In some cases they are made of wicker-work, he on top, so devised as to per their escape Another form is se or ooden spars, and having two funnel-shaped entrances at the sides These traps are baited with pieces of fish, preferably stale, and are sunk in suitable places, each attached by a line to a buoy or float
Important Lobster fisheries are carried on in Norway, Scotland, England, Ireland, Heligoland, and other parts of the coasts of Northern Europe
In the South the Lobster fishery is of less ie Crustacea, especially the Spiny Lobster, being hly esteemed
The American Lobster, as already mentioned, closely rese in the for 9, p 32) It is found on the Atlantic coast from Labrador to Cape Hatteras, but it is not abundant south of New Jersey The canning of Lobsters is a very important industry in Newfoundland, the Maritiland States
The only other species of the genus _Homarus_ (_H capensis_) is found at the Cape of Good Hope, but it is of small size and is of no economic iht of 10 pounds, although individuals of 14 pounds weight have been caught In A 20 and even 23 pounds