VII 



MOLLUSC A GENITAL ORGANS 



227 



glands, receptacula seminis, vagina, etc. Since, in hermaphrodite 

 Molluscs, both kinds of complication occur simultaneously in the 

 same genital apparatus, the most complicated arrangement is found 

 in the (hermaphrodite) Pulmonata and Opisthobranchia. 



3. Copulatory organs are wanting in many Molluscs, such as the 

 Amphineura (see below), nearly all Diotocardia, the Scaphopoda, and 

 all Lamellibranchia. They are present in the Monotocardia, the 

 Pulffioiutta, Opisthobranchia, and Cephalopoda. In the Gastropoda, in 

 the nuchal region, to the right, there is a male apparatus, consisting 

 sometimes of a freely projecting muscular penis, sometimes of an 

 organ which can be protruded or evaginated through the genital 

 aperture. In the Cephalopoda, this is a definite arm in the male, 

 which is specially modified (hectoeotilised), sometimes in a very 

 remarkable manner, and which plays a more or less important part in 

 copulation. 



B. Special. 



a. Gonads. (1) Amphineura. The long hermaphrodite gland of Proneomenia 

 and allied forms has been called paired. As a matter of fact it is divided into two 

 more or less distinct lateral tubes, by a median much-folded septum. In the lower 

 portion of each tube, that 



which lies next the intestine, 3-, .1 



the germinal epithelium pro- 

 duces spermatozoa, in the 

 upper portion eggs. Pos- 

 teriorly, these tubes sepa- 

 rate for a certain distance, 

 and open as a pair of dis- 

 tinct ducts into the anterior 

 end of the pericardium. 



The male or female gonad 

 of the Chitonidce lies as a 

 long unpaired sac on the 

 dorsal side of the intestine, in 

 front of and partly under the 

 pericardium. In the ovary, 

 numerous pear-shaped tubes 

 (Fig. 187) project from the 

 epithelial wall into the 

 cavity. Each of these tubes FIG. 1ST. Section through the wall of the ovary of Chiton 

 is a stalked follicle, with egg (diagram after Haller). 1, Eggs at different stages of develop- 

 cells surrounded by follicular ment ' 2 > ei inal epithelium ; 3, egg sac or tubes ; 4, follicular 

 cells. These follicles are epithelium ; 5, egg tube after the discharge of the egg. 

 found in all sizes and at all stages of development. Each egg is at first a simple 

 ovarial epithelial cell, which is distinguished by its size from the surrounding 

 epithelial cells. As it grows and becomes more and more rich in yolk, it sinks 

 down under the ovarial epithelium, bulging out this latter towards the ovarial 

 cavity, and thus forming a young follicle. The wall of the pear-shaped testicle 

 also rises into its cavity in the form of numerous folds, in which the epithelium 

 becomes inultilaminar, and produces the mother cells of the spermatozoa. 



The fact that the gonad of Chiton has two ducts makes it probable that it was 



