MOLLUSOA 



145 



ducts continuous with tlie 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? 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 ccelom, 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 

 Chaetopod 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 

 Chsetopoda, the Gephyrsea, 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 (oSts, 

 travail), and not as derived from the common arrangement 

 of adaptation *i a nephridium to the genital efferent func- 

 tion ( = nephrodinic).i 



The functional oviduct of Nautilus forms an albumini- 

 parous gland as a diverticulum, which appears to corre- 

 spond to a dilatation in the male duct, which succeeds the 

 testis itself, and is called the " accessory gland." 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 Ommastrephsp, 

 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 Ccelomate animals are, according to this 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 (fSios) pore heing developed. In each of 

 these latter groups the pore may he— (1) devoid of a duct, (2) provided 

 with a duct which is unattached to the gonad and opens into the hody- 

 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 Nephrogonaducts 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 (Cheetopoda) this formation of " sperm-ropes " 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 structiu-es, opening some way 

 behind the anus in Nautilus (101, g.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 

 ruode 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 "autoplasts." The autoplasts eventually 

 form a layer of fusiform cells (fig. 121, (7), h; fig. 122, m, 

 and fig. 123, ps), — the "yelk-membrane" which 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, to). 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 well as to the muscles, 

 coelom, and skeleto-trophic tissues, and to the mid-portion 

 of the alimentary canal vsdth 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 



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