Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/710

Rh 682 MOLLUSCA [CEPHALOPODA. ducts continuous with the tunic of the gonad itself occur viz., in Nematoid worms, in Arthropoda, and in Teleostean fishes, besides Mollusca there is an absence of definite knowledge as to the mode of development of the duct. It seems, however, from such facts as have been ascer tained that the gonad lies at first freely in the coelom, and that the duct develops in connexion with the genital pore, and attaches itself to the embryonic gonad, or to the capsule which grows around it. The question then arises as to the nature of the pore. In other groups of animals we find that the pore, and funnel or tube connected with it by which the genital products are conveyed to the exterior, is a modified nephridium (usually a pair, one right and one left). Is it possible that this is also the case where the duct very early becomes united to the gonad, and even gives rise to the appearance of a tubular ovary or testis 1 Probably this is the case in Teleostean fishes (see Huxley s observations on the oviducts of the smelt, 44) ; but it seems to be a tenable position that in other cases, including the Mollusca, the genital pore is a simple opening in the body-wall leading into the body-cavity or crelom, such as we find on the dorsal surface of the earth-worm, which has become specialized for the extrusion of the genital products. Possibly, as in Nemertine and Chastopod worms, the condition preceding the development of these definite genital pores was one in which a temporary rupture of the body-wall occurred at the breeding season, and this temporary aperture has gradually become perma nent. The absence of genital pores in Patella, and some Lamellibranchs which make use of the nephridia for the extrusion of their genital products, suggests that the very earliest Mollusca or their forefathers were devoid of genital ducts and pores. In no Mollusca, however, is the nephrid ium used in the same way as a genital duct as it is in the Chaetopoda, the Gephyraea, and the Vertebrata ; for the open mouth of the nephridium in Mollusca leads into the pericardial space, and it is not through this space and this mouth that the genital products of any Mollusca enter the nephridium (except perhaps in Neomenia), although it is by this mouth that the genital products enter the nephridium in the former classes above named. Hence the arrangement in Patella, &c., is to be looked upon as a special development from the simpler condition when the Mollusca brought forth by rupture ( = schizodinic, from ciSts, travail), and not as derived from the common arrangement of adaptation of a nephridium to the genital efferent func tion ( = nephrodinic). 1 The functional oviduct of Nautilus forms an albumini- parous gland as a diverticulum, Avhich appears to corre spond to a dilatation in the male duct, which succeeds the testis itself, and is called the &quot; accessory gland.&quot; The male duct has a second dilatation (Needham s sac), and then is produced in the form of a large papilla. In Dibranchs the genital ducts are but little more elaborated. They are ciliated internally. In female Octopoda, in Ommastrephes, and in one male Octopod (Eledone moschata) the genital ducts are paired, opening right and left of the anus. But in all other Dibranchs a single genital duct only is deve loped, viz., that of the left side, and leads from the genital capsule or chamber of the gonad to an asymmetrically-placed pore. In the male Dibranchs the genital duct is coiled and provided with a series of glandular dilatations and 1 Coelomate animals are, according to tliis nomenclature, either Schizodinic or Porodinic. The Porodinic group is divisible into Ne- phrodinic and Idiodinic, in the former the nephridium serving as a pore, in the latter a special (rSios) pore being developed. In each of these latter groups the pore maybe (1) devoid of a duct, (2) provided with a duct which is unattached to the gonad and opens into the body- cavity, (3) provided with a duct which fuses with the gonad. The genital ducts of Idiodinic forms may be called Idiogonaducts, as dis tinguished from the Nephrogonuducts of nephrodinic forms. receptacles. These are connected with the formation of the spermatophores. In the Siphonopoda the spermatic fluid does not flow as a liquid from the genital pore, but the spermatozoa are made up into little packets before extrusion. In other Mollusca (Pulmonata) and in other animals (Chsetopoda) this formation of &quot; sperm-ropes &quot; is known, but in the Siphonopoda it attains its highest development. Exceedingly complicated structures of a cylindrical form (sometimes an inch in length) are formed in the male genital duct by a secretion which embeds and cements together the spermatozoa. They are formed in Nautilus as well as in Dibranchs, the actual manner in which their complicated structure is produced being not easily con jectured. Accessory glands not forming part of the oviduct, but furnishing the material for enclosing the eggs in an elastic envelope, are found as paired structures, opening some way behind the anus in Nautilus (101, ff.n.) and in the Di branchs. They are known as the nidamental glands. In the female Sepia they are particularly large and prominent, and are accompanied by a second smaller pair. Reproduction and Development. The details of sexual congress and of the actual fertilization of the egg are quite unknown in Nautilus, and imperfectly in the Dibranchs and the Pteropoda. Allusion has already been made to the subject in connexion with the hectocotylized arm. The mature eggs of Nautilus are unknown, as well as the appear ance which they present when deposited. In the Dibranchs the eggs are always very large ; in some cases the amount of food-yelk infused into the original egg-cell is so great as to give it the size of a large pea. This results in that mode of development which is only known outside this class among the Vertebrata ; it is discoblastic. The proto plasm of the fertilized egg-cell segregates to one pole of the egg, and there undergoes cell-division, resulting in the formation of a disc of cleavage cells (fig. 121, (1)) resem bling the cicatricula of the hen s egg, which subsequently spreads over and invests the whole egg (fig. 121, (2)). For details of this process we must refer the reader to other works (45, 46) ; but it may here be noted that in addition to the layer of cleavage cells, which consists of more than one stratum of cells in the future embryonic area as opposed to the yelk-sac area, additional cells are formed in the mass of residual yelk apparently by an independent process of segregation, each cell having a separate origin, whence they are termed &quot;autoplasts.&quot; The autoplasts eventually form a layer of fusiform cells (fig. 121, (7), h; fig. 122, m; and fig. 123, ps), the &quot;yelk-membrane&quot; Avhich everywhere rests upon and encloses the residual yelk. The cleavage cells form a single layer on the yelk-sac area and two layers on the embryonic area, an outer layer one cell deep (fig. 122, ep), and an inner the middle layer of the three which is often thick and many cells deep (fig. 122, m). There is great difficulty here in identifying the layers with the three typical layers of other animal embryos, except in regard to the outermost, which corresponds with the epiblast of Vertebrates in many respects. The middle layer, however, gives rise to the nerve-ganglia as Avell as to the muscles, coslom, and skeleto-trophic tissues, and to the mid-portion of the alimentary canal with its hepatic diverticula, the liver (see fig. 121, (7) and explanation, where the origin of the mid-gut as a vesicle r is seen). It is clearly, therefore, something more than the mesoblast of the Vertebrate, giving rise, as it does, to important organs formed both by epiblast and hypoblast in other animals. Lastly, the yelk- membrane, though corresponding to the Vertebrate hypo- blast in position and structure, furnishes no part of the alimentary tract, but disappears when the yelk is com pletely absorbed. In fact, the developmental phenomena in Sepia, Loligo, and Octopus are profoundly perturbed by the excessive proportion of food-yelk. Balfour has shown