380 COMPARATIVE AXATOMY. 



extends even on to the smaller roots of these veins. Owing to this 

 arrangement, this secreting apparatus has relations to the venous 

 current of blood which passes to the gills ; it has therefore the 

 same relation as the excretory organ of the Lamellibranchiata and 

 Cephalophora. 



It is less certainly proved that the sacs which contain the excre- 

 tory venous appendages have any internal communications. While 

 some authors assert that there is such a connection with the blood- 

 vascular system, and especially with the pericardial sinus, others 

 deny it. These organs still, therefore, require much elucidation. 



Hancock, A., On the Structure aucl Homologies of the Renal Organ in the 

 Nudibranchiatc Mollusca. Transact. Linnean Soc. Vol. XXIV. 



Generative Organs. 



§ 293. 



Reproduction is never effected in the Mollusca by any of those 

 asexual modes which we saw in the Arthropoda to be ultimately due 

 to sexual differentiation. It is exclusively dependent on the complete 

 activity of both sets of generative organs. In several divisions the 

 two organs have been seen to arise from different layers of the 

 embryo; the male having been found to have relations to the 

 ectoderm, and the female to the endoderm. These organs are fairly 

 distinct in the different classes of the Mollusca, so that it is only 

 possible to derive them from a ground-form common to the whole 

 group, by looking for this form at a very low stage of differentiation. 



In the Placophora there is an unpaired germ-gland, from 

 which paired efferent ducts lead to the laterally and posteriorly 

 placed genital pores. Owing to the presence of separate efferent 

 ducts, the arrangement is of a higher grade than it is in the Lamelli- 

 branchiata. In most, the sexes appear to be distinct. 



In the Lamellibranchiata, individuals in which the two sexes 

 are united are only found in a few diverse genera, or even in 

 isolated species ; they probably represent the remnant of an 

 arrangement which was formerly found in the whole class. 

 In the Oysters we find an intermediate step towards a separa- 

 tion of the sexes, inasmuch as these organs are not active at 

 the same time in the same individual; but the male and female 

 organs are alternately so. The germ-glands are paired and 

 placed at the sides ; they also open separately. They generally 

 occupy a large portion of the coclom, and are often intimately con- 

 nected with other organs. 



Gradational differences can be made out in the relations of the 

 two kinds of germ-glands in hermaphrodite forms, which indi- 

 cate the lines along which the sepai\ation of the sexes has been 

 effected. In some (e.g. Ostrea) the germ-gland is a hermaphrodite 

 organ in the fullest sense of the word. The follicles which produce 



