34 COMPARATIVE ANATOMY^ 



the testes and the openings of the sperm ducts, and there 

 undergo their further development. 



The sperm-sacs, into which the sperm mother-cells are shed, 

 vary considerably in form and size at different seasons of the 

 year. When fully developed in the autumn months they are 

 conspicuous white structures with three pairs of lateral elon- 

 gated lobes projecting at the sides of the oesophagus. They 

 must be regarded as a special portion of the ccelomic cavity 

 which has been cut off by the growth of a horizontal partition 

 from the septa in segments 10 and n. Each sperm-sac may 

 be regarded as consisting of two parts, a spacious median 

 unpaired sac, distinguished as the sperm-reservoir, and paired 

 lateral appendages of these, distinguished as the sperm-sacs 

 proper. 



The anterior sperm-reservoir lies in the ventral part of 

 the loth somite, and its four corners are produced into 

 horns, which are the paired sperm-sacs. The antero-lateral 

 horns seem to project through the septum separating the 

 loth from the Qth somite, and to lie in the cavity of the 

 latter. But really they are covered by a pocket-like fold of 

 the septum, and therefore must be regarded as belonging 

 to the loth somite. Similarly, the postero-lateral horns push 

 the wall of the septum separating the loth from the nth 

 somite before them and seem to lie in the nth somite. 

 The posterior seminal reservoir lies in somite n, and has 

 only one pair of sperm-sacs connected with it ; these spring 

 from the postero-lateral angles and push through the septum 

 behind so as to project into the i2th and even into the 

 1 3th somite. The cavities of the horns or sperm-sacs proper 

 are divided up by partitions into numerous chambers, but the 

 cavities of the median seminal reservoirs are not thus divided. 



Immediately behind the testes in each seminal reservoir is 

 a pair of large ciliated funnels whose walls are thrown into 

 numerous pleats like the folded filter papers used in chemical 

 experiments. These are the openings of the sperm-ducts, and 

 from their shape they are often called the ciliated rosettes. 

 The sperm-duct from each ciliated rosette of the anterior pair 

 passes through the septum dividing the loth from the nth 

 somite, runs outwards and backwards as a fine convoluted 

 tube in the ventral body-wall, and then turns backwards, 

 runs with a straight course through the next three somites, 



