PARASITISM AND SYMBIOSIS
by this kind of trocar (Fig. 35 V) the cypridian cellular mass is, so to speak, injected into the body cavity of the crab. Delage named this larval stage the kentrogon. The cypris larva is thus changed into a small cellular mass, naked and undifferentiated, and now within the body of the crab. There has taken place, then, a very great regression, beginning in the free-living stages and particularly from the ancestral cypris in which there were already the rudiments of all the essential organs of the adult cirripede. Sacculina will remain as an internal parasite of the crab for a very long time, according to Delage about twenty months, during which the crab completes its growth. Its history during this period was elucidated by G. Smith,* who confirmed 274 and, in 1906, definitely placed beyond dispute the facts set out by Delage, which had been contested, particularly by Giard, on account of their peculiar nature. The internal, undifferentiated Sacculina undertakes a regular migration within the crab, from the point of entry, which may be anywhere, to the position on the abdomen where the external Sacculina is regularly found. Using Sacculina in Inachus mauritdnicus (=1. scorpio), Smith succeeded in fiqding the different stages of this migration which takes place along the length of the intestine, from the anterior region, where the paired ca:ca are given off, to almost opposite the unpaired abdominal ca:cum, where it stops (Fig. 36). During this time the parasite is composed of a shapeless, lobed mass from which prolongations are pushed out, constituting the beginning of the radical system. At a certain moment, towards the end of migration, there becomes differentiated in the central part, from which the first roots are given off, a sort of tumour, or nucleus, which is the beginning of the actual Sacculina. The parasite, having arrived at the abdomen of the host, finally takes up its place on the ventral surface of the intestine, opposite the unpaired posterior crecum. Within the nucleus there form, by a new differentiation, as Smith showed (and not, as Delage supposed, froPl rudiments present in the cypris stage), the definitive organs (pallial cavity, genital glands, nerve ganglia, etc.). Thus there is constituted the internal Sacculina. It presses against the ventral wall of the * Geoffrey Smith, who was consIdered to be one of the best English biologIsts of his generation, was kIlled In 1917, at the battle of the Somme. 90 ADAPTATION TO PARASITISM IN THE CRUSTACEA crab's abdomen and by. its contact produces a necrosis of the parietal muscles and of the epidermis and then a softening of the chitin over a small disc of two to three millimetres. This disc gives way, or else the crab moults, and Sacculina is then external; it now grows rapidly. A parallel type of evolution has been found in Peltogaster (ScWmkewitch, Smith); in other much rarer genera it has not yet been studied.
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The processes which constitute the development of the internal Sacculina (dedifferentiation and migration, then new differentiation) can only be the result of a progressive evolution (evidently
Figure 36. Internal stage of Sacculina. S, in the course of its migration along the gut of the crab, with the differentiation of the root system, r; ca, anterior creca, and cp, posterior crecum of the crab's intestine; n, nucleus (future external Sacculina) (after Geoffrey Smith). determined by the conditions of parasitism), of a more or less rapid type, with a succession of stages in the past which still remain entirely unknown to us. Perhaps the genera other than Sacculina and Peltogaster, whose development has not yet been studied, will throw light on these stages. * * Smith observed that a clrripede, Anelasma squalicola, attached to the skm of a shark (Spinax) possesses roothke prolongatIons buried in the integument of the host; but these are merely organs of attachment and the animal differs from the Rhizocephala III having a well-developed ahmentary canal. It cannot possIbly be considered to be one of the Rhizocephala. On the other hand, SmIth considers that a parasIte attached to the ventral surface of the isopod Calathura branchiata IS probably a primitive member of the Rhizocephala, perhaps lackmg roots. This animal, whIch was named Duplorbis calathurce, is still very little known, and has unfortunately been found only once, in Greenland.
GREGARIOUS RHIZOCEPHALA 91
The external sac of the Rhizocephala seems clearly to constitute the differentiated part of the individual, the system of roots playing only a trophic role. It is interesting, in this connection, to quote here an observation of Ch. Perez 266 on Peltogaster paguri. On the root system he saw a recurrent branch slowly form, and after the descent of the Peltogaster (the roots were watched for several years) aneurisms were formed on these recurrent branches and in the aneurisms there developed ovaries whose oocytes matured but could not be shed. The question arises oLknowing whether these late radical ovarian structures Figure 37. Thompsoma sp. on Synalphells brucei (after F. A. Potts). actually derive from the primordial genital cells of the individual or whether they are a straightforward new differentiation of root tissue, separate from the germ plasm. Coutiere 234 thought that in a gregarious genus which he called Thylacoplethus, found by him on the Alpheidre, he had discovered a primitive form of Rhizocephala with direct development, not migrating into the interior of the host. Each individual, he thought, must have developed at the point where it was found,
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LUMINOUS CEPHALOPODS
present on the surface of the egg when it is laid, and between the layers of the shell. They would thus be transmitted from one generation to the next, would be localized and would multiply in the accessory nidamental gland, which would be a specific receptacle for them. Cultures of these various bacilli have been obtained (the bacteriological study was made by Zirpolo) and those from the yellow ducts are luminescent. Now, Pierantoni has observed that in female cuttlefish at the mating season the ventral surface is luminescent, a fact that had not been noticed earlier. Let us consider the cephalopods with ventral luminous organs. Pierantoni was able to study them under favourable
Figure 73. Sections of luminous organs. A, of Rondeletia minor; E, of Sepio/a intermedia (after Pierantoni); ep, epidermis; Ie, lens; pg, pigment; rf, reflector; sf, luminous substance. circumstances in the Sepiolidre, in which these organs were discovered rather recently (in Rondeletia, Sepiola, Sepietta oweniana). The accessory nidamental gland (present only in the female) has just two kinds of ducts (white and red). The light organs (Fig. 73) which, in the female, occupy the central part of the gland, are formed of yellow ducts. It seems very probable that the light organs are a specialized part of this gland in which the yellow ducts are concentrated. In addition, there is a reflector below, formed at the expense of muscular tissue, and, above, a lens develbped from connective tissue. The' ducts of the light organ, greatly dilated (Fig. 73B), are packed with a finely granular mass, composed, according to Pierantoni, of bacteria and constituting the actual luminous substance. Taking 248 SYMBIOSIS the most careful precautions, Pierantoni and Zirpolo obtained cultures which they considered to originate from these corpuscles. In cuttlefish broth they form a white film, magnificently luminescent and emerald green, which illuminates the whole liquid when it is stirred. Pierantoni stresses the precautions employed against contamination and the differences in appearance between the cultured bacteria and those luminous ones which are commonly to be met with on the skin or muscles of cuttlefish and of dead fish. He concll\des, then, that the luminescence of the Sepiolidce must be due to symbiotic bacteria localized in the light organ, which must itself be a result of the differentiation of the accessory nidamental gland of cuttlefish and squids. He found that this light organ functions in two ways: by internal illumination of its substance (symbiotic bacteria), or by the emission of the contents of its ducts into the surrounding water, which then itself becomes luminous. The photogenic bacteria are transmitted from one generation to the next when the eggs are laid. There would thus be a hereditary physiological symbiosis which is probably very general; the Italian author has also attempted to undertake research into the luminous organs of deep sea cephalopods, unfortunately difficult to obtain in good condition. First of all, he made observations on a species (Charybditeuthis) collected at Messina. The luminescent kernel of the photogenic organs here" serait toujours constitue par des cellules remplies de microorganisms transformes par adaptation a la vie intracellulaire". This interpretation is based on the study of fixed and stained material. It was not possible to make cultures and the intracellular situation of the corpuscles considered as bacteria inevitably raises questions which will be considered later on. More recently (1924, 1926), Pierantoni 519 was able to carry out similar researches m{ an abyssal myopsid cephalopod, Heteroteuthis dispar; there, 1'00, in a ventralluminous organ occurring in both sexes he found corpuscles which, according to him, are bacteria. These various results have, moreover, given rise to rather lively controversies sustained by several Italian workers (Puntoni, Mortara 501, Skowron) who dispute the reality of the bacterial character of the corpuscles in the luminous organs,
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