4 1 8 GEPHYREA 



with glandular walls which open on the one side to the exterior, 

 and on the other by means of a ciliated funnel-shaped opening 

 into the body -cavity. In Gephyrea one wall of the tube is 

 produced into a long diverticulum or sac which hangs down 

 into the body -cavity, and is usually supported by muscle -fibres 

 running to the body-wall. The lower end of the sac is broken 

 up into a number of crypts or pits, lined by large glandular cells 

 crowded with brown pigment. The pigment-granules are secreted 

 into the cavity of the sac, and leave the body through the external 

 opening ; they probably consist of the nitrogenous excreta of the 

 animal. The upper end of the sac, into which both the external 

 and internal orifices open, is usually enlarged, and its walls are 

 very muscular. As in so many other animals, the nephridia 

 serve as ducts through which the reproductive cells leave the 

 body of the parent. 



Reproductive System. The Gephyrea are bisexual. In 

 Sipunculus the testes and ovaries are found in the same position 

 in the two sexes, and are indistinguishable without microscopic 

 investigation. They each consist of small ridges situated at the 

 lower end of the ventral retractor muscles, just where the latter 

 take their origin from the longitudinal muscles of the skin. At 

 this level the cells which line the body-cavity on the inside of 

 the skin are heaped up, and become modified in the one case into 

 ova or eggs, and in the other into the mother-cells of the sperma- 

 tozoa. This method of forming the reproductive organs from 

 modified cells lining the body -cavity is very common in the 

 higher animals ; but it is seen in its simplest and least modified 

 form in the Sipunculidae. 



The eggs break away from the ovary in a very undeveloped 

 condition, but whilst floating about in the body -cavity they 

 increase in size and secrete a thick membrane around them. 

 They have a well-marked nucleus, and are oval in outline. 



The mother-cells of the spermatozoa also break away in an 

 immature condition, and complete their development in the nutri- 

 tive fluid of the body-cavity. They divide into a number of 

 spermatozoa, usually eight or sixteen, which remain in contact. 

 They each develop a tail, which projects outwards, and aids the 

 cluster in swimming along. These clusters of spermatozoa are 

 about the same size as the ova of the female, and, like them, 

 make their way into the " brown tubes." The exact way in 



